[HDU] 1016 Prime Ring Problem(DFS)

#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cstring>
using namespace std;
int n,a[30],c[30];
int is_prim(int x)
{
if(x<2) return 0;
for(int i=2;i*i<=x;i++){
if(x%i==0) return 0;
}
return 1;
}
void dfs(int k)
{
if(k==n){
if(is_prim(a[k-1]+a[0])){
for(int i=0;i<k-1;i++){
printf("%d ",a[i]);
}
printf("%d\n",a[k-1]);
}
return;
}

for(int i=2;i<=n;i++){
if(!c[i] && is_prim(i+a[k-1])){
c[i]=1;
a[k]=i;
dfs(k+1);
c[i]=0;
}
}
}

int main()
{
int num=0;

while(~scanf("%d",&n)){
memset(c,0,sizeof(c));
printf("Case %d:\n",++num);
a[0]=1;
dfs(1);
printf("\n");
}
return 0;
}