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.

class Solution {
public:
int trap(vector<int>& height) {
int n=height.size();
if(n==0) return 0;
vector<int> left(n,height[0]),right(n,height[n-1]);
for(int i=1;i<n;++i)
left[i]=max(height[i],left[i-1]);
for(int i=n-2;i>=0;--i)
right[i]=max(height[i],right[i+1]);
int sum=0,water=0;
for(int i=0;i<n;++i)
{
sum+=height[i];
water+=min(left[i],right[i]);
}
return water-sum;
}
};