leetcode[42]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!

class Solution {

public:

    int trap(int A[], int n) {

    if (A==NULL||n<3)

        return 0;

    int maxleft=0;

    int maxright=0;

    int i=0,j=n-1;

    int sum=0;

    while(i<j)

    {

        if (maxleft<A[i])

        {

            maxleft=A[i];

        }

        if (maxright<A[j])

        {

            maxright=A[j];

        }

        if (maxleft<=maxright)

        {

            sum+=maxleft-A[i];

            i++;

        }

        else

        {

            sum+=maxright-A[j];

            j--;

        }

    }

    return sum;        

    }

};