leetcode - Trapping Rain Water

 

class Solution {
public:
    int trap(int A[], int n) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        int * lr = new int [n];
        int last = 0;
        for (int i=0; i<n; i++){
            if (A[i]<last)
                lr[i]=last-A[i];
            else{
                lr[i] = 0;
                last = A[i];
            }
        }
        int * rl = new int [n];
        last = 0;
        for (int i=n-1; i>=0; i--){
            if (A[i]<last)
                rl[i]=last-A[i];
            else{
                rl[i] = 0;
                last = A[i];
            }
        }
        int sum = 0;
        for (int i=0; i<n; i++)
            sum += lr[i]<rl[i]?lr[i]:rl[i];
        delete []lr;
        delete []rl;
        return sum;
    }
};