[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!

思考：找出左边最高跟右边最高，时间复杂度O(n).

class Solution {
public:
    int trap(int A[], int n) {
        vector<vector<int> > index(n,vector<int>(2,0));
        int i,area=0;
        for(i=0;i<n;i++)
        {
            index[i][0]=index[i][1]=A[i];
        }
        for(i=1;i<n;i++)
        {
            if(index[i][0]<index[i-1][0]) index[i][0]=index[i-1][0];
        }
        for(i=n-2;i>=0;i--)
        {
            if(index[i][1]<index[i+1][1]) index[i][1]=index[i+1][1];
        }
        for(i=0;i<n;i++)
        {
            area+=min(index[i][0],index[i][1])-A[i];
        }
        return area;
    }
};