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.
Given [0,1,0,2,1,0,1,3,2,1,2,1], return 6.
O(n) time and O(1) memory
O(n) time and O(n) memory is also acceptable.
s
[LintCode] Trapping Rain Water 收集雨水
原文：http://www.cnblogs.com/grandyang/p/6166347.html