-
Notifications
You must be signed in to change notification settings - Fork 0
Expand file tree
/
Copy pathstudy-guide.html
More file actions
2653 lines (2532 loc) · 166 KB
/
Copy pathstudy-guide.html
File metadata and controls
2653 lines (2532 loc) · 166 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Coding Interview Patterns — Field Manual</title>
<style>
:root {
--bg: #f5f6fa;
--bg-grid: rgba(79,70,229,.035);
--surface: #ffffff;
--surface-2: #eef0f7;
--surface-3: #e7eaf3;
--border: #e2e5ef;
--border-strong: #ccd1e0;
--ink: #14161f;
--muted: #545a70;
--faint: #868ca1;
--accent: #4f46e5;
--accent-ink: #4338ca;
--accent-soft: rgba(79,70,229,.10);
--accent-line: rgba(79,70,229,.30);
--code-bg: #f4f5fb;
--code-ink: #2a2e45;
--code-comment: #8a8fa8;
--code-kw: #5b3fd6;
--easy: #0f9d6b; --easy-soft: rgba(15,157,107,.12);
--med: #b06f00; --med-soft: rgba(176,111,0,.13);
--hard: #d81b52; --hard-soft: rgba(216,27,82,.11);
--shadow-sm: 0 1px 2px rgba(20,22,31,.05);
--shadow: 0 1px 2px rgba(20,22,31,.04), 0 8px 28px rgba(20,22,31,.06);
--shadow-lg: 0 12px 40px rgba(20,22,31,.12);
--mono: ui-monospace, "SF Mono", SFMono-Regular, Menlo, Consolas, monospace;
--sans: system-ui, -apple-system, "Segoe UI", Roboto, Helvetica, sans-serif;
--radius: 14px;
--topbar-h: 60px;
}
:root[data-theme="dark"] {
--bg: #0a0c13;
--bg-grid: rgba(142,140,251,.045);
--surface: #12141e;
--surface-2: #1a1d2b;
--surface-3: #222639;
--border: #242838;
--border-strong: #333850;
--ink: #e9ebf4;
--muted: #a6acc2;
--faint: #6b7189;
--accent: #8e8cfb;
--accent-ink: #a8a6ff;
--accent-soft: rgba(142,140,251,.14);
--accent-line: rgba(142,140,251,.32);
--code-bg: #0d0f1a;
--code-ink: #d4d8ef;
--code-comment: #6b7091;
--code-kw: #b3acff;
--easy: #34d399; --easy-soft: rgba(52,211,153,.14);
--med: #fbbf24; --med-soft: rgba(251,191,36,.14);
--hard: #fb7185; --hard-soft: rgba(251,113,133,.14);
--shadow-sm: 0 1px 2px rgba(0,0,0,.3);
--shadow: 0 1px 2px rgba(0,0,0,.3), 0 10px 34px rgba(0,0,0,.4);
--shadow-lg: 0 16px 46px rgba(0,0,0,.5);
}
* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
margin: 0; background: var(--bg); color: var(--ink);
font-family: var(--sans); line-height: 1.6; font-size: 16px;
-webkit-font-smoothing: antialiased; text-rendering: optimizeLegibility;
background-image:
linear-gradient(var(--bg-grid) 1px, transparent 1px),
linear-gradient(90deg, var(--bg-grid) 1px, transparent 1px);
background-size: 46px 46px;
}
a { color: inherit; text-decoration: none; }
::selection { background: var(--accent-soft); }
:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; border-radius: 4px; }
/* -------------------------------------------------------------- Top bar */
.topbar {
position: sticky; top: 0; z-index: 50; height: var(--topbar-h);
display: flex; align-items: center; gap: 18px; padding: 0 22px;
background: color-mix(in srgb, var(--surface) 82%, transparent);
backdrop-filter: saturate(180%) blur(14px);
-webkit-backdrop-filter: saturate(180%) blur(14px);
border-bottom: 1px solid var(--border);
}
.logo { display: flex; align-items: center; gap: 10px; flex: none; }
.logo .mark {
width: 30px; height: 30px; border-radius: 8px; flex: none;
background: linear-gradient(135deg, var(--accent), color-mix(in srgb, var(--accent) 55%, #12d0b4));
display: grid; place-items: center; color: #fff; font-family: var(--mono);
font-weight: 700; font-size: 15px; box-shadow: var(--shadow-sm);
}
.logo .txt { display: flex; flex-direction: column; line-height: 1.1; }
.logo .txt b { font-size: 14px; letter-spacing: -.01em; }
.logo .txt span { font-family: var(--mono); font-size: 10px; letter-spacing: .12em; text-transform: uppercase; color: var(--faint); }
.search { position: relative; flex: 1; max-width: 440px; }
.search input {
width: 100%; height: 38px; padding: 0 36px 0 36px; border-radius: 10px;
border: 1px solid var(--border-strong); background: var(--surface-2); color: var(--ink);
font-family: var(--sans); font-size: 14px;
}
.search input::placeholder { color: var(--faint); }
.search .icn { position: absolute; left: 11px; top: 50%; transform: translateY(-50%); color: var(--faint); font-size: 14px; pointer-events: none; }
.search kbd {
position: absolute; right: 9px; top: 50%; transform: translateY(-50%);
font-family: var(--mono); font-size: 11px; color: var(--faint);
background: var(--surface-3); border: 1px solid var(--border); border-radius: 5px; padding: 1px 6px;
}
.difficulty { display: flex; gap: 4px; background: var(--surface-2); border: 1px solid var(--border); border-radius: 10px; padding: 3px; }
.difficulty button {
font-family: var(--mono); font-size: 11px; letter-spacing: .04em; text-transform: uppercase;
border: 0; background: transparent; color: var(--muted); padding: 5px 10px; border-radius: 7px; cursor: pointer;
}
.difficulty button.active { background: var(--surface); color: var(--ink); box-shadow: var(--shadow-sm); font-weight: 600; }
.spacer { flex: 1; }
.iconbtn {
width: 38px; height: 38px; border-radius: 10px; border: 1px solid var(--border-strong);
background: var(--surface-2); color: var(--muted); cursor: pointer; display: grid; place-items: center; font-size: 16px;
}
.iconbtn:hover { color: var(--ink); }
/* -------------------------------------------------------------- Shell */
.shell { display: grid; grid-template-columns: 288px minmax(0,1fr); max-width: 1320px; margin: 0 auto; }
.rail {
position: sticky; top: var(--topbar-h); align-self: start;
height: calc(100vh - var(--topbar-h)); overflow-y: auto;
padding: 24px 18px 60px; border-right: 1px solid var(--border);
}
.rail .progress { padding: 4px 10px 16px; }
.rail .progress .row { display: flex; justify-content: space-between; align-items: baseline; margin-bottom: 8px; }
.rail .progress .row b { font-size: 13px; }
.rail .progress .row span { font-family: var(--mono); font-size: 12px; color: var(--accent-ink); }
.rail .bar { height: 6px; border-radius: 4px; background: var(--surface-3); overflow: hidden; }
.rail .bar > i { display: block; height: 100%; width: 0; background: linear-gradient(90deg, var(--accent), color-mix(in srgb, var(--accent) 55%, #12d0b4)); transition: width .35s ease; }
.navgroup { font-family: var(--mono); font-size: 10px; letter-spacing: .16em; text-transform: uppercase; color: var(--faint); margin: 16px 10px 6px; }
.rail nav a {
display: flex; gap: 10px; align-items: center; padding: 7px 10px; border-radius: 8px;
color: var(--muted); font-size: 13.5px; transition: background .12s, color .12s;
}
.rail nav a .num { font-family: var(--mono); font-size: 11px; color: var(--faint); width: 20px; flex: none; text-align: center; }
.rail nav a .dot { width: 7px; height: 7px; border-radius: 50%; border: 1.5px solid var(--border-strong); flex: none; margin-left: auto; }
.rail nav a.done .dot { background: var(--easy); border-color: var(--easy); }
.rail nav a:hover { background: var(--surface-2); color: var(--ink); }
.rail nav a.active { background: var(--accent-soft); color: var(--accent-ink); font-weight: 600; }
.rail nav a.active .num { color: var(--accent-ink); }
.rail nav a.plain .num { opacity: .5; }
main { padding: 34px 44px 140px; min-width: 0; }
/* -------------------------------------------------------------- Hero */
.hero { padding: 14px 0 26px; border-bottom: 1px solid var(--border); margin-bottom: 8px; }
.hero .ey { font-family: var(--mono); font-size: 12px; letter-spacing: .2em; text-transform: uppercase; color: var(--accent-ink); }
.hero h1 { font-size: clamp(30px, 4.2vw, 46px); line-height: 1.05; letter-spacing: -.03em; margin: 14px 0 16px; font-weight: 820; text-wrap: balance; max-width: 17ch; }
.hero h1 .grad { background: linear-gradient(120deg, var(--accent), color-mix(in srgb, var(--accent) 45%, #10b3d6)); -webkit-background-clip: text; background-clip: text; color: transparent; }
.hero p { font-size: 16.5px; color: var(--muted); max-width: 64ch; margin: 0 0 20px; }
.hero .stats { display: flex; flex-wrap: wrap; gap: 10px; }
.stat { font-family: var(--mono); font-size: 12px; background: var(--surface); border: 1px solid var(--border); padding: 8px 13px; border-radius: 9px; color: var(--muted); box-shadow: var(--shadow-sm); }
.stat b { color: var(--ink); font-weight: 700; }
section { scroll-margin-top: calc(var(--topbar-h) + 14px); }
h2.sec { font-size: 12.5px; font-family: var(--mono); letter-spacing: .18em; text-transform: uppercase; color: var(--faint); margin: 48px 0 18px; display: flex; align-items: center; gap: 14px; }
h2.sec::after { content: ""; height: 1px; flex: 1; background: var(--border); }
/* -------------------------------------------------------------- How-to / tips */
.tips { display: grid; grid-template-columns: repeat(auto-fit, minmax(232px,1fr)); gap: 14px; }
.tip { background: var(--surface); border: 1px solid var(--border); border-radius: 12px; padding: 17px 19px; box-shadow: var(--shadow-sm); }
.tip .n { font-family: var(--mono); font-size: 11px; letter-spacing: .08em; color: var(--accent-ink); text-transform: uppercase; }
.tip h4 { margin: 7px 0 6px; font-size: 15.5px; font-weight: 700; }
.tip p { margin: 0; font-size: 13.8px; color: var(--muted); }
/* -------------------------------------------------------------- Tables */
.tablewrap { overflow-x: auto; border: 1px solid var(--border); border-radius: 12px; background: var(--surface); box-shadow: var(--shadow); }
table { border-collapse: collapse; width: 100%; font-size: 14px; min-width: 520px; }
th, td { text-align: left; padding: 12px 16px; border-bottom: 1px solid var(--border); vertical-align: top; }
th { font-family: var(--mono); font-size: 10.5px; letter-spacing: .1em; text-transform: uppercase; color: var(--faint); background: var(--surface-2); position: sticky; top: var(--topbar-h); }
tr:last-child td { border-bottom: 0; }
tbody tr { transition: background .12s; }
tbody tr:hover { background: var(--surface-2); }
td.sig { color: var(--muted); }
td code, td.sig code { background: var(--surface-2); padding: 1px 6px; border-radius: 5px; font-size: 12.5px; font-family: var(--mono); }
td .ptn { font-weight: 680; color: var(--accent-ink); }
td .ptn:hover { text-decoration: underline; }
.callout { background: var(--accent-soft); border: 1px solid var(--accent-line); padding: 13px 16px; border-radius: 10px; font-size: 14.5px; margin-bottom: 16px; }
.callout b { color: var(--accent-ink); }
/* -------------------------------------------------------------- Pattern card */
.card { background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius); box-shadow: var(--shadow); overflow: hidden; margin-bottom: 22px; transition: box-shadow .2s, border-color .2s; }
.card:hover { box-shadow: var(--shadow-lg); }
.card.studied { border-color: var(--easy); }
.card > header { padding: 22px 26px 18px; border-bottom: 1px solid var(--border); position: relative; }
.card .toprow { display: flex; align-items: center; justify-content: space-between; gap: 12px; }
.card .idx { font-family: var(--mono); font-size: 11.5px; color: var(--accent-ink); letter-spacing: .12em; }
.card.ext .idx::after { content: " · added"; color: var(--faint); }
.card h3 { margin: 6px 0 9px; font-size: 24px; letter-spacing: -.02em; font-weight: 780; }
.card .tag { color: var(--muted); font-size: 15px; margin: 0; max-width: 68ch; }
.studybtn {
flex: none; font-family: var(--mono); font-size: 11px; letter-spacing: .04em; text-transform: uppercase;
border: 1px solid var(--border-strong); background: var(--surface-2); color: var(--muted);
padding: 6px 12px; border-radius: 8px; cursor: pointer; display: flex; gap: 6px; align-items: center; white-space: nowrap;
}
.studybtn:hover { color: var(--ink); }
.card.studied .studybtn { background: var(--easy-soft); color: var(--easy); border-color: transparent; }
.studybtn .tick { font-size: 12px; }
.body { padding: 20px 26px 24px; display: flex; flex-direction: column; gap: 20px; }
.block-label { font-family: var(--mono); font-size: 11px; letter-spacing: .14em; text-transform: uppercase; color: var(--faint); margin-bottom: 10px; display: flex; align-items: center; gap: 8px; }
.block-label::before { content: ""; width: 6px; height: 6px; border-radius: 2px; background: var(--accent); }
.triggers { display: flex; flex-wrap: wrap; gap: 7px; }
.chip { font-family: var(--mono); font-size: 12px; background: var(--surface-2); border: 1px solid var(--border); color: var(--ink); padding: 5px 10px; border-radius: 20px; }
.insight { background: var(--accent-soft); border-left: 3px solid var(--accent); padding: 12px 15px; border-radius: 0 8px 8px 0; font-size: 14.5px; }
.insight b { color: var(--accent-ink); }
figure.code { margin: 0; border-radius: 10px; overflow: hidden; border: 1px solid var(--border); background: var(--code-bg); }
figure.code figcaption { font-family: var(--mono); font-size: 11px; color: var(--faint); padding: 8px 12px 8px 14px; border-bottom: 1px solid var(--border); display: flex; justify-content: space-between; align-items: center; letter-spacing: .05em; }
.copybtn { font-family: var(--mono); font-size: 10.5px; letter-spacing: .06em; text-transform: uppercase; border: 1px solid var(--border); background: var(--surface); color: var(--muted); padding: 3px 9px; border-radius: 6px; cursor: pointer; }
.copybtn:hover { color: var(--ink); }
.copybtn.copied { color: var(--easy); border-color: var(--easy); }
pre { margin: 0; overflow-x: auto; padding: 14px 16px; }
code, pre { font-family: var(--mono); font-size: 12.7px; line-height: 1.7; color: var(--code-ink); }
pre .c { color: var(--code-comment); font-style: italic; }
pre .k { color: var(--code-kw); }
.plist { display: flex; flex-direction: column; }
.prob { display: grid; grid-template-columns: 70px 1fr; gap: 14px; padding: 10px 4px; border-bottom: 1px solid var(--border); align-items: baseline; }
.prob:last-child { border-bottom: 0; }
.prob .name { font-weight: 630; font-size: 14.5px; }
.prob .note { color: var(--muted); font-size: 13.5px; display: block; margin-top: 2px; }
.prob .note code { background: var(--surface-2); padding: 1px 5px; border-radius: 4px; font-family: var(--mono); font-size: 12px; }
.pill { font-family: var(--mono); font-size: 10px; letter-spacing: .06em; text-transform: uppercase; padding: 3px 0; border-radius: 5px; text-align: center; font-weight: 700; }
.pill.easy { color: var(--easy); background: var(--easy-soft); }
.pill.med { color: var(--med); background: var(--med-soft); }
.pill.hard { color: var(--hard); background: var(--hard-soft); }
.cx { display: flex; flex-wrap: wrap; gap: 8px; }
.cx .box { font-family: var(--mono); font-size: 12.5px; background: var(--surface-2); border: 1px solid var(--border); border-radius: 8px; padding: 8px 12px; }
.cx .box span { color: var(--faint); }
.cx .box b { color: var(--ink); font-weight: 700; }
.noresults { display: none; text-align: center; color: var(--faint); font-family: var(--mono); font-size: 14px; padding: 60px 0; }
footer { margin-top: 56px; padding-top: 22px; border-top: 1px solid var(--border); color: var(--faint); font-size: 13px; }
.filter-diff .prob { display: grid; }
body[data-diff="easy"] .prob:not(.easy),
body[data-diff="med"] .prob:not(.med),
body[data-diff="hard"] .prob:not(.hard) { display: none; }
.difficulty button .cnt { margin-left: 5px; opacity: .6; }
.diffstatus {
display: none; position: sticky; top: var(--topbar-h); z-index: 40;
padding: 8px 22px; font-size: 13px; color: var(--muted);
background: color-mix(in srgb, var(--surface) 92%, transparent);
backdrop-filter: blur(8px); -webkit-backdrop-filter: blur(8px);
border-bottom: 1px solid var(--border);
}
.diffstatus b { color: var(--ink); }
.diffstatus button {
margin-left: 12px; border: 1px solid var(--border); background: var(--surface-2);
color: var(--muted); border-radius: 7px; padding: 2px 10px; font-size: 12px; cursor: pointer;
}
.diffstatus button:hover { color: var(--ink); border-color: var(--border-strong); }
.hidden { display: none !important; }
@media (max-width: 960px) {
.shell { grid-template-columns: 1fr; }
.rail { position: static; height: auto; border-right: 0; border-bottom: 1px solid var(--border); }
.rail nav { columns: 2; }
main { padding: 26px 18px 100px; }
.difficulty, .logo .txt { display: none; }
}
@media (prefers-reduced-motion: reduce) { html { scroll-behavior: auto; } * { transition: none !important; } }
@media print { .topbar, .rail, .studybtn, .copybtn { display: none !important; } .shell { grid-template-columns: 1fr; } .card { break-inside: avoid; box-shadow: none; } }
/* ANIM-CSS */
details.prob { display: block; padding: 0; border-bottom: 1px solid var(--border); }
details.prob:last-child { border-bottom: 0; }
details.prob > summary { display: grid; grid-template-columns: 70px 1fr auto; gap: 14px; align-items: baseline; padding: 10px 4px; cursor: pointer; list-style: none; }
details.prob > summary::-webkit-details-marker { display: none; }
details.prob > summary::marker { content: ''; }
details.prob > summary:hover .name { color: var(--accent-ink); }
details.prob .chev { align-self: center; color: var(--faint); font-family: var(--mono); font-size: 11px; transition: transform .18s; }
details.prob[open] .chev { transform: rotate(90deg); color: var(--accent); }
figure.code.soln { margin: 0 4px 14px; }
figure.code.soln pre { max-height: 440px; overflow: auto; }
.anim { border: 1px solid var(--border); border-radius: 11px; background: linear-gradient(180deg, var(--surface-2), var(--surface)); padding: 16px 14px 12px; }
.anim .stage { display: flex; gap: 6px; align-items: center; justify-content: center; flex-wrap: wrap; min-height: 40px; }
.anim .stage + .stage { margin-top: 6px; }
.anim .cap { text-align: center; font-family: var(--mono); font-size: 11.5px; color: var(--muted); margin-top: 12px; min-height: 15px; }
.anim .cell { width: 34px; height: 34px; border-radius: 8px; background: var(--surface); border: 1.5px solid var(--border); display: grid; place-items: center; font-family: var(--mono); font-size: 13px; font-weight: 600; color: var(--ink); position: relative; transition: transform .3s, background .3s, border-color .3s, opacity .3s, color .3s, box-shadow .3s; }
.anim .cell.win { background: var(--accent-soft); border-color: var(--accent-line); color: var(--accent-ink); }
.anim .cell.a { border-color: var(--accent); box-shadow: 0 0 0 3px var(--accent-soft); transform: translateY(-4px); color: var(--accent-ink); }
.anim .cell.b { border-color: var(--med); box-shadow: 0 0 0 3px var(--med-soft); transform: translateY(-4px); color: var(--med); }
.anim .cell.hit { background: var(--easy-soft); border-color: var(--easy); color: var(--easy); }
.anim .cell.dim { opacity: .32; }
.anim .arrow { font-family: var(--mono); color: var(--faint); font-size: 15px; transition: color .3s, transform .3s; }
.anim .arrow.flip { color: var(--accent); font-weight: 700; }
.anim .track { position: relative; height: 58px; margin: 4px 0; }
.anim .track .bar { position: absolute; height: 15px; border-radius: 5px; background: var(--accent-soft); border: 1px solid var(--accent-line); color: var(--accent-ink); font-family: var(--mono); font-size: 10px; display: grid; place-items: center; transition: all .5s ease; }
.anim .heaps { display: flex; align-items: center; justify-content: center; gap: 8px; flex-wrap: wrap; }
.anim .half { display: flex; gap: 4px; align-items: center; padding: 7px 9px; border-radius: 8px; background: var(--surface); border: 1px dashed var(--border-strong); min-height: 34px; }
.anim .half.lo { flex-direction: row-reverse; }
.anim .lbl { font-family: var(--mono); font-size: 9px; text-transform: uppercase; letter-spacing: .07em; color: var(--faint); }
.anim .pcell { font-family: var(--mono); font-size: 12px; font-weight: 600; padding: 4px 8px; border-radius: 6px; background: var(--surface-2); border: 1px solid var(--border); }
.anim .median { font-family: var(--mono); font-size: 12px; color: var(--accent-ink); font-weight: 700; padding: 0 4px; }
.anim .subsets { display: flex; flex-wrap: wrap; gap: 5px; justify-content: center; min-height: 40px; }
.anim .subset { font-family: var(--mono); font-size: 11.5px; padding: 4px 8px; border-radius: 6px; background: var(--accent-soft); border: 1px solid var(--accent-line); color: var(--accent-ink); animation: animpop .3s ease; }
@keyframes animpop { from { transform: scale(.6); opacity: 0; } to { transform: scale(1); opacity: 1; } }
.anim .lists { display: flex; flex-direction: column; gap: 5px; }
.anim .stage.out { margin-top: 10px; padding-top: 10px; border-top: 1px dashed var(--border-strong); }
.anim .grid { display: grid; gap: 4px; justify-content: center; margin: 0 auto; max-width: 340px; }
.anim .gcell { height: 27px; border-radius: 6px; background: var(--surface); border: 1px solid var(--border); display: grid; place-items: center; font-family: var(--mono); font-size: 12px; transition: all .2s; }
.anim .gcell.hide { opacity: 0; transform: scale(.5); }
.anim .gcell.a { background: var(--accent-soft); border-color: var(--accent); color: var(--accent-ink); }
.anim .gcell.hit { background: var(--easy-soft); border-color: var(--easy); color: var(--easy); font-weight: 700; }
.anim .cell.node { width: 38px; height: 38px; border-radius: 50%; }
.anim .deg { position: absolute; top: -7px; right: -7px; width: 16px; height: 16px; border-radius: 50%; background: var(--hard); color: #fff; font-size: 9px; display: grid; place-items: center; font-family: var(--mono); border: 1.5px solid var(--surface); }
.anim .cell.node.a .deg, .anim .cell.node.dim .deg { background: var(--easy); }
</style>
</head>
<body>
<div class="topbar">
<a class="logo" href="#top">
<span class="mark"></></span>
<span class="txt"><b>Patterns</b><span>Field Manual</span></span>
</a>
<div class="search">
<span class="icn">⌕</span>
<input id="search" type="search" placeholder="Search patterns, triggers, problems…" autocomplete="off" spellcheck="false" aria-label="Search">
<kbd>/</kbd>
</div>
<div class="difficulty" role="group" aria-label="Filter by difficulty">
<button data-diff="all" class="active">All</button>
<button data-diff="easy">Easy</button>
<button data-diff="med">Med</button>
<button data-diff="hard">Hard</button>
</div>
<span class="spacer"></span>
<button class="iconbtn" id="themebtn" aria-label="Toggle theme" title="Toggle light / dark">◐</button>
</div>
<div class="diffstatus" id="diffstatus" role="status"></div>
<div class="shell" id="top">
<aside class="rail">
<div class="progress">
<div class="row"><b>Your progress</b><span id="progtext">0 / 16</span></div>
<div class="bar"><i id="progbar"></i></div>
</div>
<nav id="nav">
<a class="plain" href="#how"><span class="num">?</span>How to use</a>
<a class="plain" href="#recognize"><span class="num">◎</span>Recognition decoder</a>
<div class="navgroup">Core patterns · from the book</div>
<a href="#p1"><span class="num">01</span>Sliding Window<span class="dot"></span></a>
<a href="#p2"><span class="num">02</span>Two Pointers<span class="dot"></span></a>
<a href="#p3"><span class="num">03</span>Fast & Slow Pointers<span class="dot"></span></a>
<a href="#p4"><span class="num">04</span>Merge Intervals<span class="dot"></span></a>
<a href="#p5"><span class="num">05</span>Cyclic Sort<span class="dot"></span></a>
<a href="#p6"><span class="num">06</span>LinkedList Reversal<span class="dot"></span></a>
<a href="#p7"><span class="num">07</span>Tree BFS<span class="dot"></span></a>
<a href="#p8"><span class="num">08</span>Tree DFS<span class="dot"></span></a>
<a href="#p9"><span class="num">09</span>Two Heaps<span class="dot"></span></a>
<a href="#p10"><span class="num">10</span>Subsets<span class="dot"></span></a>
<a href="#p11"><span class="num">11</span>Modified Binary Search<span class="dot"></span></a>
<a href="#p12"><span class="num">12</span>Bitwise XOR<span class="dot"></span></a>
<div class="navgroup">Completing the 16</div>
<a href="#p13"><span class="num">13</span>Top 'K' Elements<span class="dot"></span></a>
<a href="#p14"><span class="num">14</span>K-way Merge<span class="dot"></span></a>
<a href="#p15"><span class="num">15</span>0/1 Knapsack (DP)<span class="dot"></span></a>
<a href="#p16"><span class="num">16</span>Topological Sort<span class="dot"></span></a>
<div class="navgroup">Review</div>
<a class="plain" href="#cheatsheet"><span class="num">≡</span>Cheat sheet</a>
<a class="plain" href="#tips"><span class="num">★</span>Interview delivery</a>
</nav>
</aside>
<main>
<div class="hero">
<div class="ey">Coding Interview Preparation</div>
<h1>Recognize the pattern.<br><span class="grad">Recall the template.</span></h1>
<p>Nearly every array / string / tree / graph interview question is one of sixteen recurring shapes. You don't memorize solutions — you learn to spot the pattern from the prompt and reach for its skeleton. This manual is built for that: the trigger phrases that name each pattern, the one template that solves its whole family, the canonical problems to drill, and its complexity.</p>
<div class="stats">
<span class="stat"><b>16</b> patterns</span>
<span class="stat"><b>90+</b> problems</span>
<span class="stat"><b>1</b> template each</span>
<span class="stat">code in <b>JavaScript</b></span>
<span class="stat">✓ mirrored to <b>runnable files</b></span>
</div>
</div>
<section id="how">
<h2 class="sec">How to use this</h2>
<div class="tips">
<div class="tip"><div class="n">1 · Decode</div><h4>Read the prompt for signals</h4><p>Map the question to a pattern using the recognition table below. 80% of the battle is choosing the right shape in the first minute.</p></div>
<div class="tip"><div class="n">2 · Recall</div><h4>Type the skeleton</h4><p>Each pattern has one template. Internalize it until it flows from muscle memory, then adapt the condition inside the loop.</p></div>
<div class="tip"><div class="n">3 · Drill</div><h4>Solve the family</h4><p>Do the easy anchor, then the variations — same template, different check. Mark a pattern <em>studied</em> to track progress.</p></div>
<div class="tip"><div class="n">4 · Deliver</div><h4>Always state complexity</h4><p>Close every answer with time & space. It's expected, and it proves you know <em>why</em> the pattern beats brute force.</p></div>
</div>
</section>
<section id="recognize">
<h2 class="sec">Pattern recognition — the prompt decoder</h2>
<div class="callout"><b>The one skill that gets you hired:</b> translate what the interviewer <em>says</em> into which pattern you <em>reach for</em>. Scan this table whenever you're stuck.</div>
<div class="tablewrap">
<table>
<thead><tr><th style="width:54%">When the prompt says…</th><th>Reach for</th></tr></thead>
<tbody>
<tr><td class="sig">Contiguous <code>subarray</code>/<code>substring</code>, "size k", longest/shortest window, "at most K distinct"</td><td><a class="ptn" href="#p1">Sliding Window</a></td></tr>
<tr><td class="sig">Input is <b>sorted</b>; find a <code>pair</code>/<code>triplet</code>/<code>quad</code> sum; remove/dedupe <b>in place</b>; both ends</td><td><a class="ptn" href="#p2">Two Pointers</a></td></tr>
<tr><td class="sig"><b>LinkedList</b>/sequence + cycle, "does it loop", find middle, palindrome list, happy number</td><td><a class="ptn" href="#p3">Fast & Slow Pointers</a></td></tr>
<tr><td class="sig"><code>intervals</code>, "overlap", "merge", meetings, rooms, appointments, CPU load</td><td><a class="ptn" href="#p4">Merge Intervals</a></td></tr>
<tr><td class="sig">Array of <b>n numbers in [1..n]/[0..n]</b>; missing / duplicate / corrupt; "O(n), no extra space"</td><td><a class="ptn" href="#p5">Cyclic Sort</a></td></tr>
<tr><td class="sig">"<b>Reverse</b>" a list or sub-list, "in-place", reverse in groups of k, rotate</td><td><a class="ptn" href="#p6">In-place Reversal</a></td></tr>
<tr><td class="sig">Tree, "<b>level by level</b>", level order, min depth, right/left view, connect siblings, zigzag</td><td><a class="ptn" href="#p7">Tree BFS</a></td></tr>
<tr><td class="sig">Tree, "<b>root-to-leaf path</b>", path sum/sequence, count paths, diameter, max path sum</td><td><a class="ptn" href="#p8">Tree DFS</a></td></tr>
<tr><td class="sig"><b>Median</b> of a stream / window; "smallest in one part & largest in the other"; profit scheduling</td><td><a class="ptn" href="#p9">Two Heaps</a></td></tr>
<tr><td class="sig">"Find <b>all</b> subsets / permutations / combinations", generate parentheses, abbreviations</td><td><a class="ptn" href="#p10">Subsets</a></td></tr>
<tr><td class="sig"><b>Sorted</b> + "find element / index / ceiling / next", rotated sorted, "O(log n)"</td><td><a class="ptn" href="#p11">Modified Binary Search</a></td></tr>
<tr><td class="sig">"Every number appears twice except…", missing number without overflow, complement, bit flips</td><td><a class="ptn" href="#p12">Bitwise XOR</a></td></tr>
<tr><td class="sig">"<b>Top / K largest / K smallest / K most frequent</b>", "K closest", running Kth largest</td><td><a class="ptn" href="#p13">Top 'K' Elements</a></td></tr>
<tr><td class="sig">"<b>Merge K sorted</b> lists/arrays", "Kth smallest across M lists", smallest covering range</td><td><a class="ptn" href="#p14">K-way Merge</a></td></tr>
<tr><td class="sig">Choose a <b>subset under a capacity/target</b>, equal partition, "can we make sum S", count ways</td><td><a class="ptn" href="#p15">0/1 Knapsack (DP)</a></td></tr>
<tr><td class="sig"><b>Dependencies / prerequisites</b>, "can all finish", build an order, cycle in a digraph, alien dictionary</td><td><a class="ptn" href="#p16">Topological Sort</a></td></tr>
</tbody>
</table>
</div>
</section>
<h2 class="sec">The Patterns</h2>
<div id="cards">
<!-- 01 -->
<article class="card" id="p1" data-title="Sliding Window contiguous subarray substring size k longest shortest at most K distinct window">
<header>
<div class="toprow"><div class="idx">PATTERN 01</div><button class="studybtn" data-id="p1"><span class="tick">○</span><span class="lbl">Mark studied</span></button></div>
<h3>Sliding Window</h3>
<p class="tag">Compute over every contiguous window without recomputing the overlap. Expand with the right edge; when it violates a constraint, shrink from the left. Turns O(N·K) brute force into O(N).</p>
</header>
<div class="body">
<div><div class="block-label">How it works</div><div class="anim" data-anim="window"></div></div>
<div><div class="block-label">Recognize it when</div><div class="triggers"><span class="chip">contiguous subarray</span><span class="chip">substring</span><span class="chip">window of size k</span><span class="chip">longest / shortest</span><span class="chip">at most K distinct</span><span class="chip">max / min sum</span></div></div>
<div class="insight"><b>Key move:</b> keep a running aggregate (sum, frequency map). Each step add the incoming <code>windowEnd</code> element; while the window is invalid, subtract the outgoing <code>windowStart</code> element and advance start.</div>
<figure class="code"><figcaption><span>dynamic window · template</span><button class="copybtn">Copy</button></figcaption><pre><span class="k">let</span> windowStart = 0, best = 0;
<span class="k">const</span> freq = {};
<span class="k">for</span> (<span class="k">let</span> windowEnd = 0; windowEnd < s.length; windowEnd++) {
<span class="k">const</span> right = s[windowEnd];
freq[right] = (freq[right] || 0) + 1; <span class="c">// grow window</span>
<span class="k">while</span> (<span class="c">/* window is invalid */</span> Object.keys(freq).length > K) {
<span class="k">const</span> left = s[windowStart];
freq[left]--; <span class="c">// shrink from left</span>
<span class="k">if</span> (freq[left] === 0) <span class="k">delete</span> freq[left];
windowStart++;
}
best = Math.max(best, windowEnd - windowStart + 1);
}
<span class="k">return</span> best;</pre></figure>
<div><div class="block-label">Drill these</div><div class="plist">
<details class="prob easy"><summary><span class="pill easy">easy</span><div><span class="name">Maximum Sum Subarray of Size K</span><span class="note">Fixed window: add right, at size k record max, subtract left.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/01-sliding-window.js</span><button class="copybtn">Copy</button></figcaption><pre>function maxSubarraySumOfSizeK(arr, k) {
let windowSum = 0, windowStart = 0, maxSum = 0;
for (let windowEnd = 0; windowEnd < arr.length; windowEnd++) {
windowSum += arr[windowEnd];
if (windowEnd >= k - 1) {
maxSum = Math.max(maxSum, windowSum);
windowSum -= arr[windowStart++];
}
}
return maxSum;
}</pre></figure></details>
<details class="prob easy"><summary><span class="pill easy">easy</span><div><span class="name">Smallest Subarray with Sum ≥ S</span><span class="note">Variable window: shrink while sum ≥ S, track min length.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/01-sliding-window.js</span><button class="copybtn">Copy</button></figcaption><pre>function smallestSubarrayWithSum(arr, s) {
let windowSum = 0, windowStart = 0, minLen = Infinity;
for (let windowEnd = 0; windowEnd < arr.length; windowEnd++) {
windowSum += arr[windowEnd];
while (windowSum >= s) {
minLen = Math.min(minLen, windowEnd - windowStart + 1);
windowSum -= arr[windowStart++];
}
}
return minLen === Infinity ? 0 : minLen;
}</pre></figure></details>
<details class="prob med"><summary><span class="pill med">med</span><div><span class="name">Longest Substring with K Distinct</span><span class="note">Shrink while map size > K. Fruits-into-Baskets is this with K=2.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/01-sliding-window.js</span><button class="copybtn">Copy</button></figcaption><pre>function longestSubstringKDistinct(str, k) {
let windowStart = 0, maxLen = 0;
const freq = {};
for (let windowEnd = 0; windowEnd < str.length; windowEnd++) {
const right = str[windowEnd];
freq[right] = (freq[right] || 0) + 1;
while (Object.keys(freq).length > k) {
const left = str[windowStart++];
if (--freq[left] === 0) delete freq[left];
}
maxLen = Math.max(maxLen, windowEnd - windowStart + 1);
}
return maxLen;
}
function fruitsIntoBaskets(fruits) {
return longestSubstringKDistinct(fruits.join(''), 2);
}</pre></figure></details>
<details class="prob hard"><summary><span class="pill hard">hard</span><div><span class="name">No-repeat Substring</span><span class="note">Map char → last index; jump start to <code>max(start, last+1)</code>.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/01-sliding-window.js</span><button class="copybtn">Copy</button></figcaption><pre>function longestNoRepeatSubstring(str) {
let windowStart = 0, maxLen = 0;
const lastIndex = {};
for (let windowEnd = 0; windowEnd < str.length; windowEnd++) {
const right = str[windowEnd];
if (right in lastIndex) {
windowStart = Math.max(windowStart, lastIndex[right] + 1);
}
lastIndex[right] = windowEnd;
maxLen = Math.max(maxLen, windowEnd - windowStart + 1);
}
return maxLen;
}</pre></figure></details>
<details class="prob hard"><summary><span class="pill hard">hard</span><div><span class="name">Longest Substring after K Replacements</span><span class="note">Track <code>maxRepeat</code>; shrink when <code>windowLen − maxRepeat > k</code>.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/01-sliding-window.js</span><button class="copybtn">Copy</button></figcaption><pre>function longestSubstringAfterReplacement(str, k) {
let windowStart = 0, maxLen = 0, maxRepeat = 0;
const freq = {};
for (let windowEnd = 0; windowEnd < str.length; windowEnd++) {
const right = str[windowEnd];
freq[right] = (freq[right] || 0) + 1;
maxRepeat = Math.max(maxRepeat, freq[right]);
// window size - most-frequent-letter count = letters we must replace
if (windowEnd - windowStart + 1 - maxRepeat > k) {
freq[str[windowStart++]]--;
}
maxLen = Math.max(maxLen, windowEnd - windowStart + 1);
}
return maxLen;
}
function longestOnesAfterReplacement(arr, k) {
let windowStart = 0, maxLen = 0, maxOnes = 0;
for (let windowEnd = 0; windowEnd < arr.length; windowEnd++) {
if (arr[windowEnd] === 1) maxOnes++;
if (windowEnd - windowStart + 1 - maxOnes > k) {
if (arr[windowStart++] === 1) maxOnes--;
}
maxLen = Math.max(maxLen, windowEnd - windowStart + 1);
}
return maxLen;
}</pre></figure></details>
<details class="prob hard"><summary><span class="pill hard">hard</span><div><span class="name">Permutation · Anagrams · Min Window Substring</span><span class="note">Pattern frequency map + <code>matched</code> counter.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/01-sliding-window.js</span><button class="copybtn">Copy</button></figcaption><pre>function findPermutation(str, pattern) {
const need = {};
for (const c of pattern) need[c] = (need[c] || 0) + 1;
let windowStart = 0, matched = 0;
for (let windowEnd = 0; windowEnd < str.length; windowEnd++) {
const right = str[windowEnd];
if (right in need && --need[right] === 0) matched++;
if (matched === Object.keys(need).length) return true;
if (windowEnd >= pattern.length - 1) {
const left = str[windowStart++];
if (left in need && need[left]++ === 0) matched--;
}
}
return false;
}
function findAnagrams(str, pattern) {
const need = {};
for (const c of pattern) need[c] = (need[c] || 0) + 1;
const result = [];
let windowStart = 0, matched = 0;
for (let windowEnd = 0; windowEnd < str.length; windowEnd++) {
const right = str[windowEnd];
if (right in need && --need[right] === 0) matched++;
if (matched === Object.keys(need).length) result.push(windowStart);
if (windowEnd >= pattern.length - 1) {
const left = str[windowStart++];
if (left in need && need[left]++ === 0) matched--;
}
}
return result;
}
function minWindowSubstring(str, pattern) {
const need = {};
for (const c of pattern) need[c] = (need[c] || 0) + 1;
let windowStart = 0, matched = 0, minLen = Infinity, subStart = 0;
for (let windowEnd = 0; windowEnd < str.length; windowEnd++) {
const right = str[windowEnd];
if (right in need && --need[right] >= 0) matched++;
while (matched === pattern.length) {
if (windowEnd - windowStart + 1 < minLen) {
minLen = windowEnd - windowStart + 1;
subStart = windowStart;
}
const left = str[windowStart++];
if (left in need && need[left]++ === 0) matched--;
}
}
return minLen === Infinity ? '' : str.substring(subStart, subStart + minLen);
}</pre></figure></details>
</div></div>
<div class="cx"><div class="box"><span>Time</span> <b>O(N)</b></div><div class="box"><span>Space</span> <b>O(1)</b>/<b>O(K)</b></div></div>
</div>
</article>
<!-- 02 -->
<article class="card" id="p2" data-title="Two Pointers sorted pair triplet quad sum remove dedupe in place both ends dutch flag">
<header>
<div class="toprow"><div class="idx">PATTERN 02</div><button class="studybtn" data-id="p2"><span class="tick">○</span><span class="lbl">Mark studied</span></button></div>
<h3>Two Pointers</h3>
<p class="tag">On <b>sorted</b> data, walk two indices instead of nesting loops. Converge from both ends toward a target, or use a slow "write" pointer and a fast "read" pointer for in-place work.</p>
</header>
<div class="body">
<div><div class="block-label">How it works</div><div class="anim" data-anim="twopointers"></div></div>
<div><div class="block-label">Recognize it when</div><div class="triggers"><span class="chip">sorted array</span><span class="chip">pair / triplet / quad sum</span><span class="chip">target sum</span><span class="chip">remove in place</span><span class="chip">both ends</span><span class="chip">O(1) space</span></div></div>
<div class="insight"><b>Key move:</b> if <code>arr[start]+arr[end]</code> is too small, <code>start++</code>; too big, <code>end--</code>. For triplets, sort then fix <code>i</code> and two-pointer the rest — always <b>skip duplicates</b>.</div>
<figure class="code"><figcaption><span>converging pointers · template</span><button class="copybtn">Copy</button></figcaption><pre>arr.sort((a, b) => a - b);
<span class="k">let</span> start = 0, end = arr.length - 1;
<span class="k">while</span> (start < end) {
<span class="k">const</span> sum = arr[start] + arr[end];
<span class="k">if</span> (sum === target) <span class="k">return</span> [start, end];
<span class="k">if</span> (sum < target) start++; <span class="c">// need bigger</span>
<span class="k">else</span> end--; <span class="c">// need smaller</span>
}</pre></figure>
<div><div class="block-label">Drill these</div><div class="plist">
<details class="prob easy"><summary><span class="pill easy">easy</span><div><span class="name">Pair with Target Sum (sorted Two Sum)</span><span class="note">The base case. Unsorted → use a hash map instead.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/02-two-pointers.js</span><button class="copybtn">Copy</button></figcaption><pre>function pairWithTargetSum(arr, target) {
let start = 0, end = arr.length - 1;
while (start < end) {
const sum = arr[start] + arr[end];
if (sum === target) return [start, end];
if (sum < target) start++;
else end--;
}
return [-1, -1];
}</pre></figure></details>
<details class="prob easy"><summary><span class="pill easy">easy</span><div><span class="name">Remove Duplicates · Squaring a Sorted Array</span><span class="note">Slow write-pointer; or fill from the back, largest-square-wins.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/02-two-pointers.js</span><button class="copybtn">Copy</button></figcaption><pre>function removeDuplicates(arr) {
let nextNonDup = 1;
for (let i = 1; i < arr.length; i++) {
if (arr[nextNonDup - 1] !== arr[i]) arr[nextNonDup++] = arr[i];
}
return nextNonDup;
}
function makeSquares(arr) {
const n = arr.length;
const squares = new Array(n).fill(0);
let start = 0, end = n - 1, pos = n - 1;
while (start <= end) {
const a = arr[start] * arr[start];
const b = arr[end] * arr[end];
if (a > b) { squares[pos--] = a; start++; }
else { squares[pos--] = b; end--; }
}
return squares;
}</pre></figure></details>
<details class="prob med"><summary><span class="pill med">med</span><div><span class="name">Triplet Sum to Zero (3Sum) & variants</span><span class="note">Fix <code>i</code>, two-pointer for <code>−arr[i]</code>. Closest & count-smaller reuse this.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/02-two-pointers.js</span><button class="copybtn">Copy</button></figcaption><pre>function tripletSumToZero(arr) {
arr.sort((a, b) => a - b);
const triplets = [];
for (let i = 0; i < arr.length - 2; i++) {
if (i > 0 && arr[i] === arr[i - 1]) continue; // skip duplicate anchors
let start = i + 1, end = arr.length - 1;
while (start < end) {
const sum = arr[i] + arr[start] + arr[end];
if (sum === 0) {
triplets.push([arr[i], arr[start], arr[end]]);
start++; end--;
while (start < end && arr[start] === arr[start - 1]) start++;
while (start < end && arr[end] === arr[end + 1]) end--;
} else if (sum < 0) start++;
else end--;
}
}
return triplets;
}
function tripletSumCloseToTarget(arr, target) {
arr.sort((a, b) => a - b);
let smallestDiff = Infinity;
for (let i = 0; i < arr.length - 2; i++) {
let start = i + 1, end = arr.length - 1;
while (start < end) {
const diff = target - arr[i] - arr[start] - arr[end];
if (diff === 0) return target;
if (Math.abs(diff) < Math.abs(smallestDiff) ||
(Math.abs(diff) === Math.abs(smallestDiff) && diff > smallestDiff)) {
smallestDiff = diff;
}
if (diff > 0) start++; else end--;
}
}
return target - smallestDiff;
}
function tripletsWithSmallerSum(arr, target) {
arr.sort((a, b) => a - b);
let count = 0;
for (let i = 0; i < arr.length - 2; i++) {
let start = i + 1, end = arr.length - 1;
while (start < end) {
if (arr[i] + arr[start] + arr[end] < target) {
count += end - start; // every element between start..end also works
start++;
} else end--;
}
}
return count;
}
function subarraysWithProductLessThan(arr, target) {
let product = 1, start = 0, count = 0;
for (let end = 0; end < arr.length; end++) {
product *= arr[end];
while (product >= target && start <= end) product /= arr[start++];
count += end - start + 1; // subarrays ending at `end`
}
return count;
}</pre></figure></details>
<details class="prob med"><summary><span class="pill med">med</span><div><span class="name">Dutch National Flag (0s/1s/2s)</span><span class="note">low / i / high; 0s to front, 2s to back in one pass.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/02-two-pointers.js</span><button class="copybtn">Copy</button></figcaption><pre>function dutchFlagSort(arr) {
let low = 0, high = arr.length - 1, i = 0;
while (i <= high) {
if (arr[i] === 0) { [arr[i], arr[low]] = [arr[low], arr[i]]; i++; low++; }
else if (arr[i] === 1) i++;
else { [arr[i], arr[high]] = [arr[high], arr[i]]; high--; }
}
return arr;
}</pre></figure></details>
<details class="prob med"><summary><span class="pill med">med</span><div><span class="name">4Sum · Backspace Compare · Min Window Sort</span><span class="note">Quads = two fixes + two pointers; backspace compares from the end.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/02-two-pointers.js</span><button class="copybtn">Copy</button></figcaption><pre>function quadrupleSumToTarget(arr, target) {
arr.sort((a, b) => a - b);
const quads = [];
for (let i = 0; i < arr.length - 3; i++) {
if (i > 0 && arr[i] === arr[i - 1]) continue;
for (let j = i + 1; j < arr.length - 2; j++) {
if (j > i + 1 && arr[j] === arr[j - 1]) continue;
let start = j + 1, end = arr.length - 1;
while (start < end) {
const sum = arr[i] + arr[j] + arr[start] + arr[end];
if (sum === target) {
quads.push([arr[i], arr[j], arr[start], arr[end]]);
start++; end--;
while (start < end && arr[start] === arr[start - 1]) start++;
while (start < end && arr[end] === arr[end + 1]) end--;
} else if (sum < target) start++;
else end--;
}
}
}
return quads;
}
function backspaceCompare(str1, str2) {
const nextValid = (str, index) => {
let backspaces = 0;
while (index >= 0) {
if (str[index] === '#') backspaces++;
else if (backspaces > 0) backspaces--;
else break;
index--;
}
return index;
};
let i = str1.length - 1, j = str2.length - 1;
while (i >= 0 || j >= 0) {
i = nextValid(str1, i);
j = nextValid(str2, j);
if (i < 0 && j < 0) return true;
if (i < 0 || j < 0) return false;
if (str1[i] !== str2[j]) return false;
i--; j--;
}
return true;
}
function minWindowSort(arr) {
let low = 0, high = arr.length - 1;
while (low < arr.length - 1 && arr[low] <= arr[low + 1]) low++;
if (low === arr.length - 1) return 0; // already sorted
while (high > 0 && arr[high] >= arr[high - 1]) high--;
let subMax = -Infinity, subMin = Infinity;
for (let k = low; k <= high; k++) {
subMax = Math.max(subMax, arr[k]);
subMin = Math.min(subMin, arr[k]);
}
while (low > 0 && arr[low - 1] > subMin) low--;
while (high < arr.length - 1 && arr[high + 1] < subMax) high++;
return high - low + 1;
}</pre></figure></details>
</div></div>
<div class="cx"><div class="box"><span>Pair</span> <b>O(N)</b></div><div class="box"><span>Triplet</span> <b>O(N²)</b></div><div class="box"><span>Quad</span> <b>O(N³)</b></div><div class="box"><span>Space</span> <b>O(1)</b></div></div>
</div>
</article>
<!-- 03 -->
<article class="card" id="p3" data-title="Fast Slow Pointers hare tortoise linked list cycle middle palindrome happy number">
<header>
<div class="toprow"><div class="idx">PATTERN 03</div><button class="studybtn" data-id="p3"><span class="tick">○</span><span class="lbl">Mark studied</span></button></div>
<h3>Fast & Slow Pointers</h3>
<p class="tag">Two pointers move at different speeds (hare & tortoise). If a cycle exists, the fast pointer laps the slow one and they meet — all in O(1) space.</p>
</header>
<div class="body">
<div><div class="block-label">How it works</div><div class="anim" data-anim="fastslow"></div></div>
<div><div class="block-label">Recognize it when</div><div class="triggers"><span class="chip">linked list cycle</span><span class="chip">does it loop?</span><span class="chip">find the middle</span><span class="chip">palindrome list</span><span class="chip">happy number</span></div></div>
<div class="insight"><b>Why it works:</b> in a loop the gap shrinks by one each step, so collision is guaranteed. To find the <b>cycle start</b>: measure length K, advance one pointer K nodes, then move both together.</div>
<figure class="code"><figcaption><span>cycle detection · template</span><button class="copybtn">Copy</button></figcaption><pre><span class="k">let</span> slow = head, fast = head;
<span class="k">while</span> (fast !== null && fast.next !== null) {
slow = slow.next; <span class="c">// 1 step</span>
fast = fast.next.next; <span class="c">// 2 steps</span>
<span class="k">if</span> (slow === fast) <span class="k">return</span> true; <span class="c">// met → cycle</span>
}
<span class="k">return</span> false; <span class="c">// fast hit the end → no cycle</span></pre></figure>
<div><div class="block-label">Drill these</div><div class="plist">
<details class="prob easy"><summary><span class="pill easy">easy</span><div><span class="name">LinkedList Cycle · Middle of the LinkedList</span><span class="note">When fast reaches the end, slow sits on the middle.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/03-fast-slow-pointers.js</span><button class="copybtn">Copy</button></figcaption><pre>function hasCycle(head) {
let slow = head, fast = head;
while (fast && fast.next) {
slow = slow.next;
fast = fast.next.next;
if (slow === fast) return true;
}
return false;
}
function middleOfList(head) {
let slow = head, fast = head;
while (fast && fast.next) { slow = slow.next; fast = fast.next.next; }
return slow;
}</pre></figure></details>
<details class="prob med"><summary><span class="pill med">med</span><div><span class="name">Start of LinkedList Cycle</span><span class="note">Length K, then two pointers K apart meet at the entry.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/03-fast-slow-pointers.js</span><button class="copybtn">Copy</button></figcaption><pre>function cycleLength(head) {
let slow = head, fast = head;
while (fast && fast.next) {
slow = slow.next;
fast = fast.next.next;
if (slow === fast) {
let cur = slow, len = 0;
do { cur = cur.next; len++; } while (cur !== slow);
return len;
}
}
return 0;
}
function findCycleStart(head) {
const len = cycleLength(head);
if (len === 0) return null;
let p1 = head, p2 = head;
for (let i = 0; i < len; i++) p2 = p2.next; // move p2 ahead by cycle length
while (p1 !== p2) { p1 = p1.next; p2 = p2.next; }
return p1;
}</pre></figure></details>
<details class="prob med"><summary><span class="pill med">med</span><div><span class="name">Happy Number</span><span class="note">"Sum of squared digits" is a sequence; unhappy numbers cycle without hitting 1.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/03-fast-slow-pointers.js</span><button class="copybtn">Copy</button></figcaption><pre>function isHappyNumber(num) {
const squareSum = (n) => {
let sum = 0;
while (n > 0) { const d = n % 10; sum += d * d; n = Math.floor(n / 10); }
return sum;
};
let slow = num, fast = num;
do {
slow = squareSum(slow);
fast = squareSum(squareSum(fast));
} while (slow !== fast);
return slow === 1;
}</pre></figure></details>
<details class="prob med"><summary><span class="pill med">med</span><div><span class="name">Palindrome LinkedList · Rearrange LinkedList</span><span class="note">Find middle, reverse the second half, compare / interleave.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/03-fast-slow-pointers.js</span><button class="copybtn">Copy</button></figcaption><pre>function reverse(head) {
let prev = null, cur = head;
while (cur) { const next = cur.next; cur.next = prev; prev = cur; cur = next; }
return prev;
}
function isPalindromicList(head) {
if (!head || !head.next) return true;
// find middle
let slow = head, fast = head;
while (fast && fast.next) { slow = slow.next; fast = fast.next.next; }
// reverse second half
let secondHead = reverse(slow);
const copy = secondHead;
let p1 = head, p2 = secondHead, isPalindrome = true;
while (p1 && p2) {
if (p1.value !== p2.value) { isPalindrome = false; break; }
p1 = p1.next; p2 = p2.next;
}
reverse(copy); // restore
return isPalindrome;
}
function reorderList(head) {
if (!head || !head.next) return head;
let slow = head, fast = head;
while (fast && fast.next) { slow = slow.next; fast = fast.next.next; }
let second = reverse(slow);
let first = head;
while (first && second) {
let tmp = first.next; first.next = second; first = tmp;
tmp = second.next; second.next = first; second = tmp;
}
if (first) first.next = null;
return head;
}</pre></figure></details>
</div></div>
<div class="cx"><div class="box"><span>Time</span> <b>O(N)</b></div><div class="box"><span>Space</span> <b>O(1)</b></div></div>
</div>
</article>
<!-- 04 -->
<article class="card" id="p4" data-title="Merge Intervals overlap meeting rooms appointments free time cpu load schedule">
<header>
<div class="toprow"><div class="idx">PATTERN 04</div><button class="studybtn" data-id="p4"><span class="tick">○</span><span class="lbl">Mark studied</span></button></div>
<h3>Merge Intervals</h3>
<p class="tag">Anything about overlapping ranges. Sort by start, then sweep once: intervals overlap when the next start ≤ current end. A min-heap on end-times handles "how many at once".</p>
</header>
<div class="body">
<div><div class="block-label">How it works</div><div class="anim" data-anim="intervals"></div></div>
<div><div class="block-label">Recognize it when</div><div class="triggers"><span class="chip">intervals</span><span class="chip">overlap</span><span class="chip">merge</span><span class="chip">meeting rooms</span><span class="chip">appointments</span><span class="chip">free time</span></div></div>
<div class="insight"><b>Six overlap cases collapse to one test:</b> after sorting by start, <code>b.start ≤ a.end</code> means overlap — merge into <code>[a.start, max(a.end, b.end)]</code>. For concurrency counting, track the smallest end-time in a <b>min-heap</b>.</div>
<figure class="code"><figcaption><span>merge overlapping · template</span><button class="copybtn">Copy</button></figcaption><pre>intervals.sort((a, b) => a[0] - b[0]);
<span class="k">const</span> merged = [];
<span class="k">let</span> [start, end] = intervals[0];
<span class="k">for</span> (<span class="k">let</span> i = 1; i < intervals.length; i++) {
<span class="k">const</span> [s, e] = intervals[i];
<span class="k">if</span> (s <= end) end = Math.max(end, e); <span class="c">// overlap → extend</span>
<span class="k">else</span> { merged.push([start, end]); [start, end] = [s, e]; }
}
merged.push([start, end]);
<span class="k">return</span> merged;</pre></figure>
<div><div class="block-label">Drill these</div><div class="plist">
<details class="prob med"><summary><span class="pill med">med</span><div><span class="name">Merge Intervals · Insert Interval</span><span class="note">Insert: skip those ending before the new start, merge the overlapping run, append the rest.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/04-merge-intervals.js</span><button class="copybtn">Copy</button></figcaption><pre>function mergeIntervals(intervals) {
if (intervals.length < 2) return intervals;
intervals.sort((a, b) => a[0] - b[0]);
const merged = [];
let [start, end] = intervals[0];
for (let i = 1; i < intervals.length; i++) {
const [s, e] = intervals[i];
if (s <= end) end = Math.max(end, e); // overlap -> extend
else { merged.push([start, end]); [start, end] = [s, e]; }
}
merged.push([start, end]);
return merged;
}
function insertInterval(intervals, newInterval) {
const merged = [];
let i = 0, [ns, ne] = newInterval;
while (i < intervals.length && intervals[i][1] < ns) merged.push(intervals[i++]);
while (i < intervals.length && intervals[i][0] <= ne) {
ns = Math.min(ns, intervals[i][0]);
ne = Math.max(ne, intervals[i][1]);
i++;
}
merged.push([ns, ne]);
while (i < intervals.length) merged.push(intervals[i++]);
return merged;
}</pre></figure></details>
<details class="prob med"><summary><span class="pill med">med</span><div><span class="name">Intervals Intersection</span><span class="note">Overlap = <code>[max(starts), min(ends)]</code>; advance whichever ends first.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/04-merge-intervals.js</span><button class="copybtn">Copy</button></figcaption><pre>function intervalsIntersection(a, b) {
const result = [];
let i = 0, j = 0;
while (i < a.length && j < b.length) {
const start = Math.max(a[i][0], b[j][0]);
const end = Math.min(a[i][1], b[j][1]);
if (start <= end) result.push([start, end]);
if (a[i][1] < b[j][1]) i++; else j++;
}
return result;
}</pre></figure></details>
<details class="prob med"><summary><span class="pill med">med</span><div><span class="name">Conflicting Appointments</span><span class="note">Sort, then any <code>start < prevEnd</code> is a conflict.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/04-merge-intervals.js</span><button class="copybtn">Copy</button></figcaption><pre>function canAttendAll(appointments) {
appointments.sort((a, b) => a[0] - b[0]);
for (let i = 1; i < appointments.length; i++) {
if (appointments[i][0] < appointments[i - 1][1]) return false;
}
return true;
}</pre></figure></details>
<details class="prob hard"><summary><span class="pill hard">hard</span><div><span class="name">Minimum Meeting Rooms · Max CPU Load</span><span class="note">Min-heap of end times; heap size = rooms / concurrent load.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/04-merge-intervals.js</span><button class="copybtn">Copy</button></figcaption><pre>function minMeetingRooms(meetings) {
if (meetings.length === 0) return 0;
meetings.sort((a, b) => a[0] - b[0]);
const endTimes = MinHeap(); // heap of end times of active meetings
let maxRooms = 0;
for (const [start, end] of meetings) {
while (!endTimes.isEmpty() && endTimes.peek() <= start) endTimes.pop();
endTimes.push(end);
maxRooms = Math.max(maxRooms, endTimes.size());
}
return maxRooms;
}
function maxCPULoad(jobs) {
jobs.sort((a, b) => a[0] - b[0]);
const heap = MinHeap((x, y) => x[1] - y[1]); // by end time
let current = 0, max = 0;
for (const job of jobs) {
while (!heap.isEmpty() && heap.peek()[1] <= job[0]) current -= heap.pop()[2];
heap.push(job);
current += job[2];
max = Math.max(max, current);
}
return max;
}</pre></figure></details>
<details class="prob hard"><summary><span class="pill hard">hard</span><div><span class="name">Employee Free Time</span><span class="note">Merge all intervals; the gaps between blocks are the free time.</span></div><span class="chev">▸</span></summary><figure class="code soln"><figcaption><span>solution · patterns/04-merge-intervals.js</span><button class="copybtn">Copy</button></figcaption><pre>function employeeFreeTime(schedules) {
const all = [];
for (const emp of schedules) for (const iv of emp) all.push(iv);
all.sort((a, b) => a[0] - b[0]);
const free = [];
let end = all[0][1];
for (let i = 1; i < all.length; i++) {
if (all[i][0] > end) { free.push([end, all[i][0]]); end = all[i][1]; }
else end = Math.max(end, all[i][1]);
}