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.

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!

public class Solution {    public int trap(int[] A) {// Start typing your Java solution below// DO NOT write main() functionint len = A.length;if(len < 3)return 0;int left = 0, right = len - 1, res = 0;int wall;while(left < right){if(A[left] <= A[right]){wall = left + 1;while(A[wall] <= A[left] && wall < right){res += A[left] - A[wall];wall++;}left = wall;}else{ // A[left] > A[right]wall = right - 1;while(A[wall] <= A[right] && wall > left){res += A[right] - A[wall];wall--;}right = wall;}}return res;}}

每次移动left和right里面小的向中间出发，才能保证另一侧有closure，移动wall注意不能超过另一侧

O(N)