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

Example 2

Input:  height = [4,2,0,3,2,5]
Output: 9

Constraints

• 1 <= n <= 2*10^4
• 0 <= height[i] <= 10^5