Number Of Boomerangs (#23)

Author: Jakub Jirous

README.md

@@ -42,3 +42,4 @@ Plus it has a gamified environment where you can gain XP, level up and unlock ac
 
 1. [Seven Boom](/src/challenges/hard/01-seven-boom/INDEX.md)
 2. [Tower of Hanoi](/src/challenges/hard/02-tower-of-hanoi/INDEX.md)
+3. [Number Of Boomerangs](/src/challenges/hard/03-number-of-boomerangs/INDEX.md)

src/challenges/hard/02-tower-of-hanoi/tower-of-hanoi.ts

@@ -2,7 +2,7 @@ export type Input = number;
 export type Output = number;
 
 /**
- *A function that takes a number of discs as an argument and returns the minimum amount of steps needed to complete the game.
+ * A function that takes a number of discs as an argument and returns the minimum amount of steps needed to complete the game.
  *
  * @param discs
  */

src/challenges/hard/03-number-of-boomerangs/INDEX.md

@@ -0,0 +1,48 @@
+# Number Of Boomerangs
+
+A boomerang is a V-shaped sequence that is either upright or upside down. 
+
+Specifically, a boomerang can be defined as: sub-array of length 3, with the first and last digits being the same and the middle digit being different.
+
+Some boomerang examples:
+
+```
+[3, 7, 3], [1, -1, 1], [5, 6, 5]
+```
+
+Create a function that returns the total number of boomerangs in an array.
+
+To illustrate:
+
+- 3 boomerangs in this sequence: `[3, 7, 3]`, `[1, 5, 1]`, `[2, -2, 2]`
+    ```
+    [3, 7, 3, 2, 1, 5, 1, 2, 2, -2, 2]
+    ```
+  
+Be aware that boomerangs can overlap, like so:
+
+- 5 boomerangs (from left to right): `[1, 7, 1]`, `[7, 1, 7]`, `[1, 7, 1]`, `[7, 1, 7]`, and `[1, 7, 1]`
+    ```
+    [1, 7, 1, 7, 1, 7, 1]
+    ```
+
+---
+
+### Examples:
+
+```
+numberOfBoomerangs([9, 5, 9, 5, 1, 1, 1]) ➞ 2
+numberOfBoomerangs([5, 6, 6, 7, 6, 3, 9]) ➞ 1
+numberOfBoomerangs([4, 4, 4, 9, 9, 9, 9]) ➞ 0
+```
+
+### Notes:
+
+- `[5, 5, 5]` (triple identical digits) is NOT considered a boomerang because the middle digit is identical to the first and last.
+
+---
+
+### Solution:
+
+- [Code](/src/challenges/hard/03-number-of-boomerangs/number-of-boomerangs.ts)
+- [Tests](/src/challenges/hard/03-number-of-boomerangs/test/number-of-boomerangs.test.ts)

src/challenges/hard/03-number-of-boomerangs/number-of-boomerangs.ts

@@ -0,0 +1,15 @@
+export type Input = number[];
+export type Output = number;
+
+/**
+ * A function that returns the total number of boomerangs in an array.
+ *
+ * @param inputArray
+ */
+export const numberOfBoomerangs = (inputArray: Input): Output => {
+  return inputArray.reduce((acc, value, index, array) => {
+    return value !== array[index + 1] && value === array[index + 2]
+      ? acc + 1
+      : acc;
+  }, 0);
+};

src/challenges/hard/03-number-of-boomerangs/test/number-of-boomerangs.test.ts

@@ -0,0 +1,28 @@
+import { Input, Output, numberOfBoomerangs } from "../number-of-boomerangs";
+
+type Cases = [Input, Output][];
+
+describe("Number Of Boomerangs", () => {
+  const cases: Cases = [
+    [[9, 5, 9, 5, 1, 1, 1], 2],
+    [[5, 6, 6, 7, 6, 3, 9], 1],
+    [[4, 4, 4, 9, 9, 9, 9], 0],
+    [[5, 9, 5, 9, 5], 3],
+    [[4, 4, 4, 8, 4, 8, 4], 3],
+    [[2, 2, 2, 2, 2, 2, 3], 0],
+    [[2, 2, 2, 2, 3, 2, 3], 2],
+    [[1, 2, 1, 1, 1, 2, 1], 2],
+    [[5, 1, 1, 1, 1, 4, 1], 1],
+    [[3, 7, 3, 2, 1, 5, 1, 2, 2, -2, 2], 3],
+    [[1, 7, 1, 7, 1, 7, 1], 5],
+    [[5, 5, 5], 0],
+  ];
+
+  test.each(cases)(
+    "for inputArray = %j output should be %d",
+    (firstArg, expectedResult) => {
+      const result = numberOfBoomerangs(firstArg);
+      expect(result).toEqual(expectedResult);
+    }
+  );
+});