[Leetcode] 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!

 Solution:

通过扫描两边数组，找到该列的左边，右边最大的值。

public class Solution {
    public int trap(int[] A) {
        int N = A.length;
        if (N < 3)
            return 0;
        int[] l2r = new int[N];
        int[] r2l = new int[N];
        int left = 0;
        int right = 0;
        for (int i = 0; i < N; ++i) {
            l2r[i] = left;
            left = Math.max(left, A[i]);
        }
        for (int i = N - 1; i >= 0; --i) {
            r2l[i] = right;
            right = Math.max(right, A[i]);
        }
        int sum=0;
        for(int i=0;i<N;++i){
            int temp=Math.min(l2r[i], r2l[i])-A[i];
            if(temp>0)
                sum+=temp;                
        }
        return sum;
    }
}