https://leetcode.com/problems/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.
使用双指针，分别从两头往最高处逼近。每次经过刷新左边和右边的最高值，该处结果等于最高值与当前值的差。
![LeetCode 刷题记录: 101. Symmetric Tree [Python]](/images/thumbnail.svg)
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