Prime Ring Problem
Time Limit: 4000/2000 MS (Java/Others) Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 20489 Accepted Submission(s): 9159

Problem Description
A ring is compose of n circles as shown in diagram. Put natural number 1, 2, ..., n into each circle separately, and the sum of numbers in two adjacent circles should be a prime.

Note: the number of first circle should always be 1.

Input
n (0 < n < 20).

Output
The output format is shown as sample below. Each row represents a series of circle numbers in the ring beginning from 1 clockwisely and anticlockwisely. The order of numbers must satisfy the above requirements. Print solutions in lexicographical order.

You are to write a program that completes above process.

Print a blank line after each case.

Sample Input
6
8

Sample Output
Case 1:
1 4 3 2 5 6
1 6 5 2 3 4

Case 2:
1 2 3 8 5 6 7 4
1 2 5 8 3 4 7 6
1 4 7 6 5 8 3 2
1 6 7 4 3 8 5 2

题目大意： 判断 1-n 相邻两个数相加的和是不是素数，输出素数环
DFS 算法

import java.io.*;
import java.util.*;
public class Main {
	public static int n;
	public static boolean mark[];
	public static int a[];
	public static PrintWriter pw;
	public static void main(String[] args) {
		Scanner sc=new Scanner(new BufferedInputStream(System.in));//输入
		pw=new PrintWriter(new BufferedOutputStream(System.out),true);//输出
		int number=1;
		while(sc.hasNextInt()){
			n=sc.nextInt();
			mark=new boolean[n+1];
			a=new int[n+1];
			mark[1]=true;
			a[0]=1;
			pw.println("Case "+number+++":");
			DFS(1);
			pw.println();
		}
	}
	public static void DFS(int len){
		
		if(len==n){
			//判断第一个元素和最后一个元素是不是素数
			if(isPrime(a[0]+a[n-1])){
				for(int i=0;i