<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> </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> </p> <p><strong>Constraints:</strong></p> <ul> <li><code>n == height.length</code></li> <li><code>1 <= n <= 2 * 10<sup>4</sup></code></li> <li><code>0 <= height[i] <= 10<sup>5</sup></code></li> </ul>