[HDU 1016]--Prime Ring Problem(回溯)

 1 #include <iostream>
 2 #include <cstring>
 3 using namespace std;
 4 
 5 #define maxn 40
 6 int vis[21], x[21], T, n;
 7 int prime[maxn] = { 1, 1, 0 };
 8 void init()
 9 {
10     int i, j;
11     for (i = 2; i <= maxn; i++){
12         if (!prime[i]){
13             for (j = 2; i*j <= maxn; j++)
14                 prime[i*j] = 1;
15         }
16     }
17 }
18 
19 void dfs(int cur){
20     if (cur == n&&!prime[1 + x[n - 1]]){
21         for (int i = 0; i < n; i++){
22             if (i) cout << ‘ ‘;
23             cout << x[i];
24         }
25         cout << endl;
26     }
27     else for (int i = 2; i <= n; i++){
28         if (!vis[i] && !prime[i + x[cur - 1]]){
29             x[cur] = i;
30             vis[i] = 1;
31             dfs(cur + 1);
32             vis[i] = 0;
33         }
34     }
35 }
36 
37 int main(){
38     init();
39     while (cin >> n){
40         cout << "Case " << ++T << ‘:‘ << endl;
41         if (n == 1)
42             cout << 1 << endl;
43         else if (n & 1)
44             cout << endl;
45         else{
46             memset(vis, 0, sizeof(vis));
47             x[0] = 1;
48             dfs(1);
49         }
50         cout << endl;//没加这一句pe来个wa,我也是醉了,各种改,无爱了~~~~
51     }
52     return 0;
53 }