Problem: Trapping Rain Water
Given n non-negative integers representing an elevation map, where the width of each bar is 1, compute how much water it is able to trap after raining.
For example, Given [0,1,0,2,1,0,1,3,2,1,2,1], return 6
From:LeetCode 42 URL:https://leetcode.com/problems/trapping-rain-water/description/