素数环

    输入正整数n，把整数1，2，3，...，n组成一个环，使得相邻两个整数和为素数，输出时从整数1开始逆时针排列。同一个环只输出一次，n <= 16

样例输入：

6

样例输出：

1 4 3 2 5 6

1 6 5 2 3 4

 

回溯法

#include <stdio.h>#include <iostream>#include <math.h>using namespace std;const int maxn = 20;int a[maxn];bool vis[maxn];bool isPrame(int n){for (int i = 2; i <= sqrt(n); ++i){if (!(n % i))return false;}return true;}void work(int n, int cur){if (cur > n){if (isPrame(a[1] + a[n])){ for (int i = 1; i <= n; ++i)printf("%d ", a[i]);printf("\n");}}else{for (int i = 1; i <= n; ++i)if (!vis[i]){a[cur] = i;if (isPrame(a[cur] + a[cur - 1])){vis[i] = 1;work(n, cur + 1);vis[i] = 0;}}}}int main(){int n;scanf("%d", &n);a[1] = 1;vis[1] = 1;work(n, 2);return 0;}