hdu-1016 Prime Ring Problem

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

经典 dfs问题：没有剪枝  218ms

#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[n]+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;}