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(vector<int>& height) {
int res = 0;
int l = 0;
int r = height.size() - 1;

while(l < r)
{
int minh = min(height[l], height[r]);
if(height[l] == minh)
while(++ l < r && height[l] < minh)
res += minh - height[l];
else
while(l < -- r && height[r] < minh)
res += minh - height[r];
}

return res;
}
};