Trapping Rain Water

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Question:

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!

Solution:

 1 class Solution { 2 public: 3     int trap(vector<int>& height) { 4     int n=height.size(); 5     int sum=0; 6     int *max_left=new int[n]; 7     int *max_right=new int[n]; 8     max_left[0]=0; 9     max_right[n-1]=0;10     for(int i=1;i<n-1;i++)11     {12         max_left[i]=max(max_left[i-1],height[i-1]);13         max_right[n-1-i]=max(max_right[n-i],height[n-i]);14     }15     for(int j=1;j<n-1;j++)16     {        17         int val=min(max_left[j],max_right[j]);18         if(val>height[j])19             sum+=val-height[j];20     }21     return sum;   22     }23 };

Trapping Rain Water