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!

// https://oj.leetcode.com/problems/trapping-rain-water/
// Author : Chao Zeng
// Date   : 2015-2-11
class Solution {
public:
    int trap(int A[], int n) {
        int ans = 0;
        int maxnum = A[0];
        int flag = 0;
        for (int i = 0; i < n; i++){
            if (maxnum < A[i]){
                maxnum = A[i];
                flag = i;
            }
        }
        
        int currentmaxnum = A[0];

        for (int i = 0; i < flag ; i++){
            if (currentmaxnum < A[i]){
                currentmaxnum = A[i];
            }

            if (A[i] < maxnum && A[i] < currentmaxnum){
                ans += currentmaxnum - A[i];
            }
        }

        currentmaxnum = A[n - 1];

        for (int i = n -1; i > flag; i--){
            if (currentmaxnum < A[i]){
                currentmaxnum = A[i];
            }

            if (A[i] < maxnum && A[i] < currentmaxnum){
                ans += currentmaxnum - A[i];
            }
        }
        return ans;
    }
};