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. Thanks Marcos for contributing this image!

 分析： 对于每一点只需要知道其左边和右边的最高山即可

class Solution {
public:
    int trap(int A[], int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
         if(n <= 2) return 0;
        
        vector<int> left(n,0);
        vector<int> right(n, 0);
        
        left[0] = A[0];
        for(int i =1; i < n;++i)
        {
          left[i] = left[i-1] > A[i] ?  left[i-1] : A[i]; 
        }
        
        right[n-1] = A[n-1];
        for(int i = n-2; i>=0 ;--i){
            right[i] = right[i+1] > A[i] ? right[i+1] : A[i];
        }
        
        int res = 0;
        for(int i = 0; i< n; i++){
        
            int min = left[i] < right[i]? left[i] : right[i];
            res += min - A[i];
        }
        
        return res;
    }
};