【LeetCode】Trapping Rain Water 2013年美团网校园招聘研发工程师笔试题

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.

The above elevation map 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. Thanks Marcos for contributing this image!

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java code : lhs 对应能装水的台阶的左边界，rhs对应右边界，在这个边界确定的范围内，对每一个A[i] ，其能装到的水的大小为 min{左边最高值 ，右边界最高值} - A[i] if ( 该值 > A[i]) .我们预处理出这样一个左边界的最高值数组lheight[] ，然后从右边扫过来即可。算法的复杂度是O(n)。