Add solution #42

Author: JoshCrozier

README.md

@@ -23,6 +23,7 @@
 31|[Next Permutation](./0031-next-permutation.js)|Medium|
 36|[Valid Sudoku](./0036-valid-sudoku.js)|Medium|
 41|[First Missing Positive](./0041-first-missing-positive.js)|Hard|
+42|[Trapping Rain Water](./0042-trapping-rain-water.js)|Hard|
 43|[Multiply Strings](./0043-multiply-strings.js)|Medium|
 49|[Group Anagrams](./0049-group-anagrams.js)|Medium|
 50|[Pow(x, n)](./0050-powx-n.js)|Medium|

solutions/0042-trapping-rain-water.js

@@ -0,0 +1,35 @@
+/**
+ * 42. Trapping Rain Water
+ * https://leetcode.com/problems/trapping-rain-water/
+ * Difficulty: Hard
+ *
+ * Given `n` non-negative integers representing an elevation map where the width
+ * of each bar is `1`, compute how much water it can trap after raining.
+ */
+
+/**
+ * @param {number[]} height
+ * @return {number}
+ */
+var trap = function(height) {
+  const leftMax = [];
+  const rightMax = [];
+  let result = 0;
+
+  leftMax[0] = height[0];
+  rightMax[height.length - 1] = height[height.length - 1];
+
+  for (let i = 1; i < height.length; i++) {
+    leftMax[i] = Math.max(height[i], leftMax[i - 1]);
+  }
+
+  for (let i = height.length - 2; i > -1; i--){
+    rightMax[i] = Math.max(height[i], rightMax[i + 1]);
+  }
+
+  for (let i = 0; i < height.length; i++) {
+    result += Math.min(leftMax[i], rightMax[i]) - height[i];
+  }
+
+  return result;
+};