hdu 1016 Prime Ring Problem (dfs)

1 #include <iostream>
2 #include <cstring>
3 #include <cstdio>
4 bool isp[40], vis[20];                        //isp[]素数标记数组
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)                            //调用dfs(1)
18 {
19     if (cur == n && isp[a[0]+a[n-1]])        //递归边界
20     {
21         for (int i = 0; i < n - 1; ++i)
22             printf("%d ", a[i]);
23         printf("%d\n", a[n-1]);
24     }
25     else
26     {
27         for (int i = 2; i <= n; ++i)        //尝试放置剩余的数
28             if (!vis[i] && isp[i+a[cur-1]])
29             {
30                 a[cur] = i;
31                 vis[i] = 1;                //标记
32                 dfs(cur + 1);
33                 vis[i] = 0;                //清除标记
34             }
35     }
36 }
37 int main()
38 {
39     for (int i = 2; i <= 40; ++i)            //生成素数标记
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 }