素数环

素数环一定是奇数偶数交替出现，所以一定不能是奇数个。

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 1e6 + 10, INF = 0x3f3f3f3f, MOD = 1E9 + 7;
int n, a[N], cnt;
bool st[N];

bool isp(int n) {
    for (int i = 2; i <= n / i; i++)
        if (n % i == 0) return 0;
    return n > 1;
}
void dfs(int u) {
    if (cnt >= 10) exit(0);
    if (u > n) {
        if (isp(a[n] + a[1])) {
            for (int i = 1; i <= n; i++)
                cout << a[i] << " ";
            cout << endl;
            cnt++;
        }
        return;
    }
    for (int i = 2; i <= n; i++) {
        if (!st[i] && isp(a[u - 1] + i)) {
            a[u] = i, st[i] = 1, dfs(u + 1);
            st[i] = 0;
        }
    }
}
int main(int argc, char* argv[]) {
    ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
    cin >> n;
    if (n & 1) exit(0);
    a[1] = st[1] = 1, dfs(2);
    return 0;
}