UVa524 Prime Ring Problem (回溯法)

 1 #include <iostream>
 2 #include <cstring>
 3 using namespace std;
 4 
 5 int n, isp[35], A[20], vis[20];
 6 
 7 int is_prime(int x) {
 8     for (int i = 2; i < x; i++)
 9         if (!(x % i)) return 0;
10     return 1;
11 }
12 
13 void dfs(int cur) {
14     if (cur == n)
15         if (isp[A[0] + A[n - 1]]) {
16             for (int i = 0; i < n; i++) {
17                 if (i != 0) cout << " ";
18                 cout << A[i];
19             }
20             cout << endl;
21         } else return;
22     else for (int i = 2; i <= n; i++)
23         if (!vis[i] && isp[i + A[cur - 1]]) {
24             A[cur] = i;
25             vis[i] = 1;
26             dfs(cur + 1);
27             vis[i] = 0;
28         }
29 }
30 
31 int main() {
32     int kase = 0;
33     A[0] = 1;
34     for (int i = 2; i < 32; i++) isp[i] = is_prime(i);
35     while (cin >> n && n) {
36         memset(vis, 0, sizeof(vis));
37         if (kase++) cout << endl;
38         cout << "Case " << kase << ":" << endl;
39         dfs(1);
40     }
41     return 0;
42 }