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;
}
};