[leetcode-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:
The number at the ith position is divisible by i.
i is divisible by the number at the ith position.
Now given N, how many beautiful arrangements can you construct?
Example 1:
Input: 2
Output: 2
Explanation:
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:
N is a positive integer and will not exceed 15.

思路：

回溯法。。。

int counts(int n,vector<int>& intvec)
     {
         if (n <= 0) return 1;
         int res = 0;
         for (int i = 0; i < n;i++)
         {
             if (intvec[i]%n ==0 || n %intvec[i] ==0)
             {
                 swap(intvec[i], intvec[n - 1]);
                 res += counts(n - 1, intvec);
                 swap(intvec[i], intvec[n - 1]);
             }
         }
         return res;
     }
     int countArrangement(int N)
     {
         vector<int> intvec;
         for (int i = 0; i < N; i++)intvec.push_back(i + 1);
         return counts(N, intvec);         
     }

参考：

https://discuss.leetcode.com/topic/79921/my-c-elegant-solution-with-back-tracking