Prime Ring Problem dfs

A ring is compose of n circles as shown in diagram. Put natural number 1, 2, ..., n into each circle separately, and the sum of numbers in two adjacent circles should be a prime.

Note: the number of first circle should always be 1.

Inputn (0 < n < 20).

OutputThe output format is shown as sample below. Each row
represents a series of circle numbers in the ring beginning from 1
clockwisely and anticlockwisely. The order of numbers must satisfy the
above requirements. Print solutions in lexicographical order.

You are to write a program that completes above process.

Print a blank line after each case.

Sample Input

6
8

Sample Output

Case 1:
1 4 3 2 5 6
1 6 5 2 3 4

Case 2:
1 2 3 8 5 6 7 4
1 2 5 8 3 4 7 6
1 4 7 6 5 8 3 2
1 6 7 4 3 8 5 2

代码：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <map>
#include <cmath>
int n,ans[]={};
using namespace std;
int vis[];
bool isnum(int x)
{
    if(x<=)return false;
    for(int i=;i<=sqrt(x);i++)
    {
        if(x%i==)return false;
    }
    return true;
}
void dfs(int k,int last)
{
    if(k==n)
    {
        if(!isnum(ans[]+ans[n-]))return;
        printf("%d",ans[]);
        for(int i=;i<n;i++)
            printf(" %d",ans[i]);
        putchar('\n');
        return;
    }
    for(int i=;i<=n;i++)
    if(!vis[i]&&isnum(last+i))
    {
        vis[i]=;
        ans[k]=i;
        dfs(k+,i);
        vis[i]=;
    }
}
int main()
{
    int k=;
    while(cin>>n)
    {
        vis[]=;
        printf("Case %d:\n",++k);
        dfs(,);
        putchar('\n');
    }
}