hdu 1016 Prime Ring Problem (dfs)

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

#include <iostream>
#include <cstring>
#include <cstdio>
bool isp[40], vis[20];                        //isp[]素数标记数组
int n, a[20];
bool is_prime(int n)                          //n >= 2
{
    bool flag = 1;
    for (int i = 2; i * i <= n; ++i)
        if (n % i == 0)
        {
            flag = 0;
            break;
        }
    return flag;
}
void dfs(int cur)                            //调用dfs(1)
{
    if (cur == n && isp[a[0]+a[n-1]])        //递归边界
    {
        for (int i = 0; i < n - 1; ++i)
            printf("%d ", a[i]);
        printf("%d\n", a[n-1]);
    }
    else
    {
        for (int i = 2; i <= n; ++i)        //尝试放置剩余的数
            if (!vis[i] && isp[i+a[cur-1]])
            {
                a[cur] = i;
                vis[i] = 1;                //标记
                dfs(cur + 1);
                vis[i] = 0;                //清除标记
            }
    }
}
int main()
{
    for (int i = 2; i <= 40; ++i)            //生成素数标记
        isp[i] = is_prime(i);
    int cnt = 0;
    while (scanf("%d", &n) != EOF)
    {
        ++cnt;
        memset(vis, 0, sizeof(vis));
        for (int i = 0; i < n; ++i)
            a[i] = i + 1;
        printf("Case %d:\n", cnt);
        dfs(1);
        printf("\n");
    }
    return 0;
}