hdu-1016 Prime Ring Problem

#include<cstdio>
#include<cstring>
int n,vis[21],a[21];
bool is_prime(int x)
{
if(x==2||x==3) return 1;
if(x==1) return 0;
for(int i=2;i*i<=x;i++)
if(x%i==0) return 0;
return 1;
}
void dfs(int x)
{
if(x==n&&is_prime(a
+a[1]))
{
for(int i=1;i<=n;i++)
{
if(i!=n) printf("%d ",a[i]);
else printf("%d\n",a[i]);
}
return;
}
for(int i=2;i<=n;i++)
{
if(!vis[i]&&is_prime(a[x]+i))
{
vis[i]=1;
a[x+1]=i;
dfs(x+1);
vis[i]=0;
}
}
}

int main()
{
int i=1;
while(scanf("%d",&n)!=EOF)
{
printf("Case %d:\n",i++);
memset(vis,0,sizeof(vis));
memset(a,0,sizeof(a));
a[1]=1;
vis[1]=1;
dfs(1);
printf("\n");
}
return 0;
}