Trapping Rain Water

Given an array height representing an elevation map where the width of each bar is 1, compute how much water can be trapped after it rains.

Example

Input: height = [0,1,0,2,1,0,1,3,2,1,2,1] Output: 6

Constraints

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