hdoj 1016 Prime Ring Problem

 1 #include <stdio.h>
 2 #include <string.h>
 3 int n;
 4 int vis[30], a[30];
 5 int judge(int s)
 6 {
 7     int flag = 1;
 8     for(int i = 2; i <= s/2; i++)
 9     {
10         if(s%i == 0)
11         {
12             flag = 0;
13             break;
14         }
15     }
16     return flag;
17 } 
18 void dfs(int s, int cnt)
19 {
20     int i, j;
21     if(cnt  == n+1 && judge(a[1]+a[n]))
22     {
23         for(j = 1; j < n+1; j++)
24         {
25             if(j != 1)
26                 printf(" ");
27             printf("%d", a[j]);
28         }
29         printf("\n");
30         return ;
31     }
32     for(i = 1; i <= n; i++)
33     {
34         if(judge(i + s) && !vis[i])
35         {
36             a[cnt] = i;
37             vis[i] = 1;
38             dfs(i, cnt + 1);
39             vis[i] = 0;
40             
41         }
42     }
43     return ;
44 }
45 int main()
46 {
47     int num = 1;
48     while(~scanf("%d", &n))
49     {
50         printf("Case %d:\n", num++);
51         memset(vis, 0, sizeof(vis));
52         vis[1] = 1;
53         a[1] = 1;
54         dfs(1, 2);
55         printf("\n");
56     }
57     return 0;
58 }