#533 - Trapping Rain Water

challenges/533-trapping-rain-water/README.md

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+Write a function `trap` that calculates how much rainwater can be trapped between bars in a histogram after raining.
+
+Given an array of non-negative integers `height` where each element represents the elevation at that point, compute how much water it can trap.
+
+### Signature
+
+```ts
+function trap(height: number[]): number;
+```
+
+### Example
+
+```ts
+trap([0,1,0,2,1,0,1,3,2,1,2,1]); // 6
+trap([4,2,0,3,2,5]); // 9
+```
+
+### Explanation
+
+Each bar's trapped water is determined by the **minimum of the highest bars** to its left and right, minus its own height.
+
+For example, in `[0,1,0,2,1,0,1,3,2,1,2,1]`, at index 5 (`height = 0`):
+
+* Left max = 2
+* Right max = 3
+* Water trapped = `min(2, 3) - 0 = 2`.
+
+The total water trapped is the sum across all indices.
+
+### Constraints
+
+* `n == height.length`
+* `0 ≤ n ≤ 10⁵`
+* `0 ≤ height[i] ≤ 10⁴`
+* The algorithm should run in `O(n)` time and `O(1)` extra space.

challenges/533-trapping-rain-water/info.yml

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+difficulty: hard
+title: Trapping Rain Water
+type: question
+template: javascript
+tags: javascript, arrays, two-pointers
+author:
+  github: jsartisan
+  name: Pawan Kumar
+  avatar_url: https://avatars.githubusercontent.com/u/6636360?v=4
+published_date: '2025-10-07'
+

challenges/533-trapping-rain-water/template.template.md

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+```js index.js 
+/**
+ * @param {number[]} height
+ * @return {number}
+ */
+export function trap(height) {
+  // TODO: Implement me
+  return 0;
+}
+```
+
+```js index.test.js 
+import { trap } from './index';
+
+describe('trap (JS)', () => {
+  it('handles empty array', () => {
+    expect(trap([])).toBe(0);
+  });
+
+  it('handles single bar', () => {
+    expect(trap([5])).toBe(0);
+  });
+
+  it('handles no trapped water', () => {
+    expect(trap([1,2,3,4,5])).toBe(0);
+  });
+
+  it('handles simple valley', () => {
+    expect(trap([2,0,2])).toBe(2);
+  });
+
+  it('handles complex case', () => {
+    expect(trap([0,1,0,2,1,0,1,3,2,1,2,1])).toBe(6);
+  });
+
+  it('handles another complex case', () => {
+    expect(trap([4,2,0,3,2,5])).toBe(9);
+  });
+});
+```
+
+```ts index.ts 
+export function trap(height: number[]): number {
+  // TODO: Implement me
+  return 0;
+}
+```
+
+```ts index.test.ts 
+import { trap } from './index';
+
+describe('trap (TS)', () => {
+  it('returns 0 for empty array', () => {
+    expect(trap([])).toBe(0);
+  });
+
+  it('returns 0 for increasing heights', () => {
+    expect(trap([1,2,3,4])).toBe(0);
+  });
+
+  it('returns 2 for [2,0,2]', () => {
+    expect(trap([2,0,2])).toBe(2);
+  });
+
+  it('returns 6 for classic example', () => {
+    expect(trap([0,1,0,2,1,0,1,3,2,1,2,1])).toBe(6);
+  });
+
+  it('returns 9 for [4,2,0,3,2,5]', () => {
+    expect(trap([4,2,0,3,2,5])).toBe(9);
+  });
+});
+```
+
+