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(n <=2) return 0;
        int t = 0;
        int l = A[0], r=A[n-1];
        int bgn = 1, end = n-2;
        while(bgn <= end){
            if(l <  r){
                t += std::max(l - A[bgn], 0);
                l = std::max(l, A[bgn]);
                bgn++;
            }
            else{
                t += std::max(r - A[end], 0);
                r = std::max(r, A[end]);
                end--;
            }
        }
        return t;
    }
};

leetcode - Trapping Rain Water