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. **Example 1:** ![](https://assets.leetcode.com/uploads/2018/10/22/rainwatertrap.png) Input: height = [0,1,0,2,1,0,1,3,2,1,2,1] Output: 6 Explanation: 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. **Example 2:** Input: height = [4,2,0,3,2,5] Output: 9 **Constraints:** * `n == height.length` * `1 <= n <= 2 * 104` * `0 <= height[i] <= 105`