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;        
    }
};