hdu 1016 Prime Ring Problem (dfs)

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

 1 #include <iostream> 2 #include <cstring> 3 #include <cstdio> 4 bool isp[40], vis[20];                        //isp[]素數標記數組 5 int n, a[20]; 6 bool is_prime(int n)                          //n >= 2 7 { 8     bool flag = 1; 9     for (int i = 2; i * i <= n; ++i)10         if (n % i == 0)11         {12             flag = 0;13             break;14         }15     return flag;16 }17 void dfs(int cur)                            //調用dfs(1)18 {19     if (cur == n && isp[a[0]+a[n-1]])        //遞迴邊界20     {21         for (int i = 0; i < n - 1; ++i)22             printf("%d ", a[i]);23         printf("%d\n", a[n-1]);24     }25     else26     {27         for (int i = 2; i <= n; ++i)        //嘗試放置剩餘的數28             if (!vis[i] && isp[i+a[cur-1]])29             {30                 a[cur] = i;31                 vis[i] = 1;                //標記32                 dfs(cur + 1);33                 vis[i] = 0;                //清除標記34             }35     }36 }37 int main()38 {39     for (int i = 2; i <= 40; ++i)            //產生素數標記40         isp[i] = is_prime(i);41     int cnt = 0;42     while (scanf("%d", &n) != EOF)43     {44         ++cnt;45         memset(vis, 0, sizeof(vis));46         for (int i = 0; i < n; ++i)47             a[i] = i + 1;48         printf("Case %d:\n", cnt);49         dfs(1);50         printf("\n");51     }52     return 0;53 }