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;

    }

};