42. Trapping Rain Water / Hard / JavaScript

yeongrok/0042-trapping-rain-water/0042-trapping-rain-water.js

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+/**
+ * @param {number[]} height
+ * @return {number}
+ */
+var trap = function (height) {
+    if (height.length <= 2) return 0;
+
+    let left = 0;
+    let right = height.length - 1;
+    let maxLeftHeight = 0;
+    let maxRightHeight = 0;
+    let totalWater = 0;
+
+    while (left < right) {
+        if (height[left] < height[right]) {
+            if (height[left] >= maxLeftHeight) {
+                maxLeftHeight = height[left];
+            } else {
+                totalWater += maxLeftHeight - height[left];
+            }
+            left++;
+        } else {
+            if (height[right] >= maxRightHeight) {
+                maxRightHeight = height[right];
+            } else {
+                totalWater += maxRightHeight - height[right];
+            }
+            right--;
+        }
+    }
+
+    return totalWater;
+};

yeongrok/0042-trapping-rain-water/README.md

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+<h2><a href="https://leetcode.com/problems/trapping-rain-water/">42. Trapping Rain Water</a></h2><h3>Hard</h3><hr><div><p>Given <code>n</code> non-negative integers representing an elevation map where the width of each bar is <code>1</code>, compute how much water it can trap after raining.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+<img src="https://assets.leetcode.com/uploads/2018/10/22/rainwatertrap.png" style="width: 412px; height: 161px;">
+<pre><strong>Input:</strong> height = [0,1,0,2,1,0,1,3,2,1,2,1]
+<strong>Output:</strong> 6
+<strong>Explanation:</strong> The above elevation map (black section) is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre><strong>Input:</strong> height = [4,2,0,3,2,5]
+<strong>Output:</strong> 9
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>n == height.length</code></li>
+	<li><code>1 &lt;= n &lt;= 2 * 10<sup>4</sup></code></li>
+	<li><code>0 &lt;= height[i] &lt;= 10<sup>5</sup></code></li>
+</ul>
+</div>