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.

public class Solution {    int result = 0;    public int countArrangement(int N) {        if(N<=0){            return 0;        }                ArrayList<Integer> list = new ArrayList<Integer>();        dfs(N, 1, list);        return result;    }        public void dfs(int N, int position,ArrayList<Integer> list){        if(position > N){            result++;            return;        }        for(int i=1; i<=N; i++){            if((i%position == 0 || position%i == 0) && (!list.contains(i))){                list.add(i);                dfs(N, position+1, list);                list.remove(list.size()-1);            }        }    }}