day15.md

Day 15 — Strings & StringBuilder

DSA Partner Challenge | Week 3: Strings

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📌 Topic of the Day

Strings & StringBuilder — the most important topic. Finally.

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🎥 Resources

Language	Resource
☕ Java	Strings + StringBuilder
🐍 Python	Live Session 1 + Video 2
⚙️ C++	Striver A2Z DSA playlist — Strings section

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🧠 Part 1 — Java: Strings

Key Facts

• String is an object, not a primitive
• Strings are IMMUTABLE — any modification creates a new object
• Always use .equals() for comparison — never ==
• String literals go into the String Pool for reuse

Most-Used Methods

String s = "hello world";

s.length()              // 11
s.charAt(0)             // 'h'
s.substring(6, 11)      // "world"
s.indexOf("world")      // 6
s.contains("hello")     // true
s.replace("hello","hi") // "hi world"
s.toLowerCase()         // "hello world"
s.toUpperCase()         // "HELLO WORLD"
s.trim()                // removes leading/trailing spaces
s.split(" ")            // ["hello", "world"]
s.equals("hello world") // true
s.toCharArray()         // ['h','e','l','l','o',' ','w','o','r','l','d']
String.valueOf(42)      // "42"
Integer.parseInt("42")  // 42

Why Immutability Matters

// BAD — O(n²): creates 1000 String objects
String result = "";
for (int i=0; i<1000; i++) result += i;

// GOOD — O(n): one object, modified in-place
StringBuilder sb = new StringBuilder();
for (int i=0; i<1000; i++) sb.append(i);
String result = sb.toString();

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🧠 Part 2 — Java: StringBuilder

StringBuilder sb = new StringBuilder();
sb.append("Hello");        // "Hello"
sb.append(" World");       // "Hello World"
sb.insert(5, "!");         // "Hello! World"
sb.delete(5, 6);           // "Hello World"
sb.reverse();              // "dlroW olleH"
sb.charAt(0);              // 'd'
sb.length();               // 11
sb.toString();             // convert to String

// Common pattern — reverse a String:
String reversed = new StringBuilder(s).reverse().toString();

Feature	String	StringBuilder
Mutable	No	Yes
Thread safe	Yes	No
Concatenation	+ (slow in loops)	append() (fast)
Use case	Comparisons, fixed	Building in loops

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🧠 Part 3 — Python: Strings

s = "hello world"

len(s)                # 11
s[0]                  # 'h'
s[-1]                 # 'd'
s[6:11]               # 'world'
s[::-1]               # 'dlrow olleh' — reverse
s.lower()             # 'hello world'
s.upper()             # 'HELLO WORLD'
s.strip()             # remove whitespace
s.split(' ')          # ['hello', 'world']
' '.join(['a','b'])   # 'a b'
s.replace('l','r')    # 'herro worrd'
s.find('world')       # 6  (-1 if not found)
s.count('l')          # 3
'world' in s          # True
s.startswith('he')    # True
s.isdigit()           # False
f"Value: {42}"        # 'Value: 42'
str(42)               # '42'
int('42')             # 42

Fast string building in Python:

# BAD — O(n²)
result = ''
for i in range(1000): result += str(i)

# GOOD — O(n) — collect then join
parts = [str(i) for i in range(1000)]
result = ''.join(parts)

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🧠 Part 4 — C++: Strings

#include <string>
using namespace std;

string s = "hello";

s.length()              // 5
s[0]                    // 'h'  ← mutable! s[0]='H' works!
s.substr(1, 3)          // "ell"  (start, LENGTH — not end!)
s.find("ll")            // 2  (string::npos if not found)
s.replace(0, 2, "HE")  // "HEllo"
s += " world"           // "hello world"  ← mutable
s.insert(5, "!")        // "hello! world"
s.erase(5, 1)           // "hello world"
s.empty()               // false
stoi(s)                 // string to int
to_string(42)           // int to string

// Character functions (#include <cctype>)
tolower('A')            // 'a'
toupper('a')            // 'A'
isalpha('a')            // true
isdigit('3')            // true

💡 C++ std::string is mutable — no StringBuilder needed. s.substr(pos, len) takes LENGTH not end index (different from Java!).

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📊 Quick Comparison

Feature	Java	Python	C++
Mutable	No	No	Yes
Compare	.equals()	==	==
Length	.length()	len(s)	.length()
Char at i	.charAt(i)	s[i]	s[i]
Slice	.substring(a,b)	s[a:b]	.substr(a,b-a)
Reverse	new StringBuilder(s).reverse()	s[::-1]	reverse(s.begin(),s.end())
Int→String	String.valueOf(n)	str(n)	to_string(n)
String→Int	Integer.parseInt(s)	int(s)	stoi(s)
Fast build	StringBuilder	''.join(list)	string (mutable)

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💻 Practice Problems — 7 Easy LeetCode Problems

Q1. Defanging an IP Address — LC #1108 Easy

return address.replace('.', '[.]')

Q2. Shuffle String — LC #1528 Easy

res = [''] * len(s)
for i, idx in enumerate(indices): res[idx] = s[i]
return ''.join(res)

Q3. Goal Parser Interpretation — LC #1678 Easy

return command.replace('()', 'o').replace('(al)', 'al')

Q4. Count Items Matching a Rule — LC #1773 Easy

idx = {'type':0, 'color':1, 'name':2}[ruleKey]
return sum(1 for item in items if item[idx] == ruleValue)

Q5. Sorting the Sentence — LC #1859 Easy

words = s.split()
res = [''] * len(words)
for w in words: res[int(w[-1]) - 1] = w[:-1]
return ' '.join(res)

Q6. Check if Two String Arrays are Equivalent — LC #1662 Easy

return ''.join(word1) == ''.join(word2)

Q7. To Lower Case — LC #709 Easy

return s.lower()

Manual approach (understand ASCII):

// A=65, a=97, difference=32
// lowercase = uppercase + 32
for (char c : s.toCharArray())
    sb.append(c>='A'&&c<='Z' ? (char)(c+32) : c);

ASCII to know: 'A'=65, 'Z'=90, 'a'=97, 'z'=122, '0'=48, '9'=57

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✅ Checklist Before You Sleep

• I understand String immutability in Java and Python
• I always use .equals() for String comparison in Java
• I know when to use StringBuilder (loops, building strings in Java)
• I know ''.join(list) is the StringBuilder equivalent in Python
• I know C++ std::string is mutable — no StringBuilder needed
• I know ASCII values for A, a, 0 and the gap of 32
• I solved all 7 LeetCode problems
• I can convert between String and int in all 3 languages

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💬 Community

Solved all 7? Drop a 🔥 in the community.

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Next up → Day 16: String problems — Medium difficulty