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]
, return6
.

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& height) {
        int result = 0;
        int max_index = 0;
        int n = height.size();
        for(int i=0;iheight[i])
              result += second - height[i];
            else
              second = height[i];
        }
        second = 0;
        for(int i = n-1;i>max_index;i--)
        {
            if(second>height[i])
              result += second - height[i];
            else
              second = height[i];
        }
        return result;
    }
};