42\. Trapping Rain Water

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.

**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`
*   <code>1 <= n <= 2 * 10<sup>4</sup></code>
*   <code>0 <= height[i] <= 10<sup>5</sup></code>