526. Beautiful Arrangement

Suppose you have N integers from 1 to N. We define a beautiful arrangement as an array that is constructed by these N numbers successfully if one of the following is true for the ith position (1 ≤ i ≤ N) in this array:

1. The number at the ith position is divisible by i.
2. i is divisible by the number at the ith position.

Now given N, how many beautiful arrangements can you construct?

Example 1:

Input: 2Output: 2Explanation: 
The first beautiful arrangement is [1, 2]:
Number at the 1st position (i=1) is 1, and 1 is divisible by i (i=1).
Number at the 2nd position (i=2) is 2, and 2 is divisible by i (i=2).
The second beautiful arrangement is [2, 1]:
Number at the 1st position (i=1) is 2, and 2 is divisible by i (i=1).
Number at the 2nd position (i=2) is 1, and i (i=2) is divisible by 1.

Note:

1. N is a positive integer and will not exceed 15.

class Solution {public:    int count = 0;    int countArrangement(int N) {        bool visited[N+1]={0};        calculate(N,1,visited);        return count;    }    void calculate(int N, int pos, bool visited[]) {        if (pos > N)            count++;        for (int i = 1; i <= N; i++) {            if (!visited[i] && (pos % i == 0 || i % pos == 0)) {                visited[i] = true;                calculate(N, pos + 1, visited);                visited[i] = false;            }        }    }};