feat: add Trapping Rain Water

Author: dukeluo

README.md

@@ -19,3 +19,4 @@ https://neetcode.io/roadmap
 | 167 | [Two Sum II - Input Array Is Sorted](https://leetcode.com/problems/two-sum-ii-input-array-is-sorted/) | Medium     | [TypeScript](./TypeScript/167.two-sum-ii-input-array-is-sorted.ts) | Two Pointers     |
 | 15  | [3Sum](https://leetcode.com/problems/3sum/)                                                           | Medium     | [TypeScript](./TypeScript/15.3sum.ts)                              | Two Pointers     |
 | 11  | [Container With Most Water](https://leetcode.com/problems/container-with-most-water/)                 | Medium     | [TypeScript](./TypeScript/11.container-with-most-water.ts)         | Two Pointers     |
+| 42  | [Trapping Rain Water](https://leetcode.com/problems/trapping-rain-water/)                             | Hard       | [TypeScript](./TypeScript/42.trapping-rain-water.ts)               | Two Pointers     |

TypeScript/42.trapping-rain-water.ts

@@ -0,0 +1,38 @@
+function trap(height: number[]): number {
+  const maxLeft = new Array(height.length).fill(0);
+  const maxRight = new Array(height.length).fill(0);
+
+  for (let i = 1; i < height.length; i++) {
+    maxLeft[i] = Math.max(maxLeft[i - 1], height[i - 1]);
+  }
+  for (let i = height.length - 2; i >= 0; i--) {
+    maxRight[i] = Math.max(maxRight[i + 1], height[i + 1]);
+  }
+
+  return height.reduce((s, h, i) => {
+    const v = Math.min(maxLeft[i], maxRight[i]) - h;
+    return v > 0 ? s + v : s;
+  }, 0);
+}
+
+function trap2(height: number[]): number {
+  let l = 0;
+  let r = height.length - 1;
+  let ml = height[l];
+  let mr = height[r];
+  let s = 0;
+
+  while (l < r) {
+    if (ml < mr) {
+      l++;
+      ml = Math.max(ml, height[l]);
+      s += ml - height[l];
+    } else {
+      r--;
+      mr = Math.max(mr, height[r]);
+      s += mr - height[r];
+    }
+  }
+
+  return s;
+}