Prime Ring Problem

http://acm.hdu.edu.cn/showproblem.php?pid=1016

素数结，简单dfs，只是前面的prime我开得太小，起先只开到21，可是题目算的是两数之和，最高可达38

#include<stdio.h>
#include<string.h>
int prime[40]={0,0,1,1,0,1,0,1,0,0,0,1,0,1,0,0,0,1,0,1,0,0,0,1,0,0,0,0,0,1,0,1,0,0,0,0,0,1,0,0};
int ans[21],vis[21],n;
int print(int x)
{
    int i;
    for(i=1;i<=x;i++)
    if(i==1)
    printf("%d",ans[i]);
    else printf(" %d",ans[i]);
    printf("\n");
}
int dfs(int x)
{
    int i; 
    if(x==n&&prime[ans[x]+ans[1]])//如果最后一个跟第一个加起来是素数，就输出 
    print(x);
    else 
    {
         for(i=2;i<=n;i++)
         {
             if(prime[ans[x]+i] && !vis[i])//相邻 
             {
                 ans[x+1]=i;
                 vis[i]=1;//赋值进行深搜 
                 dfs(x+1);
                 vis[i]=0;//回溯后要赋为未访问状态 
             }
         }
    }
}
int main()
{
    int t=1;
    while(~scanf("%d",&n))
    {
       printf("Case %d:\n",t++);
       memset(vis,0,sizeof(vis));
       vis[1]=1;
       ans[1]=1;
       dfs(1);
       printf("\n");
    }
    return 0;
}