public class Solution
extends Object
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:**
**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