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!

 

 1 class Solution {
 2 public:
 3     int trap(vector<int>& height) {
 4         int n=height.size();
 5         int result=0;
 6         vector<int> left(n),right(n);
 7         for(int i=0;i<n;i++)
 8         {
 9             left[i]=i?max(left[i-1],height[i]):height[i];
10         }
11         for(int j=n-1;j>=0;j--)
12         {
13             right[j]=(n-1-j)?max(right[j+1],height[j]):height[j];
14         }
15         for(int k=0;k<n;k++)
16         {
17             result+=min(left[k],right[k])-height[k];
18         }
19         return result;
20     }
21 };