素数环

输入正整数n，把整数1，2，3，。。。，n 组成一个环，使得相邻连个整数之间和均为素数。

输出时从整数1开始逆时针排列。同一个环应恰好输出一次。n<=16。

样例输入：6

样例输出：1 4 3 2 5 6

                 1 6 5 2 3 4

解：

public class Main {

	static int n;
	static int[] a;
	static int[] book;

	static void fun(int cur) {
		if (cur == n && su(a[0] + a[n - 1])) {
			for (int i = 0; i < n; i++)
				System.out.print(a[i] + " ");
			System.out.println();
		} else
			for (int i = 2; i <= n; i++)
				if (su(i + a[cur - 1]) && book[i - 1] == 0) {
					a[cur] = i;
					book[i - 1] = 1;
					fun(cur + 1);
					book[i - 1] = 0;
				}
	}

	static boolean su(int num) {
		if (num == 1)
			return false;
		for (int i = 2; i <= Math.sqrt(num); i++)
			if (num % i == 0)
				return false;
		return true;
	}

	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		n = in.nextInt();
		a = new int[n];
		book = new int[n];
		a[0] = 1;
		book[0] = 1;
		fun(1);
	}
}