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!

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;    }};