codevs 1031 质数环

#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<algorithm>
#include<cmath>
using namespace std;
int a[100];bool b[100];
int n;
void print();
bool pd(int,int);
int dfs(int q)
{
    for(int i=2;i<=n;i++)
    {
        if((!b[i])&&pd(a[q-1],i)==1)
        {
            a[q]=i;
            b[i]=1;
            if(q==n&&pd(a[q],a[1])==1)print();
            else dfs(q+1);
            b[i]=0;
        }
    }
}
bool pd(int x,int y)
{
    int k=2,i=x+y;
    while (k<=sqrt(i)&&i%k!=0) k++;
    if (k>sqrt(i)) return 1;
    else return 0;

}
void print()
{
    for(int i=1;i<=n;i++)
    {
        cout<<a[i]<<" ";
    }
    cout<<endl;
}
int main()
{
    a[1]=1;
    cin>>n;if(n%2==1){cout<<endl;return 0;}
    dfs(2);
    return 0;
}