九度oj 题目1459：Prime ring problem

题目描述：

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.

输入：

n (1 < n < 17).

输出：

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.

样例输入：

6
8

样例输出：

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

提示：

用printf打印输出。

此题应该用深搜来求解，代码如下

 #include <cstdio>
 #include <iostream>
 #include <cstring>
 #include <queue>
 #include <algorithm>
 using namespace std;

 int n;

 int isPrime[];
 void getPrime() {
     memset(isPrime, , sizeof(isPrime));
     for (int i = ; i <= ; i++) {
         if (isPrime[i] == ) {
             for (int j = i*; j <= ; j += i) {
                 isPrime[j] = ;
             }
         }
     }

 }

 int flag[];
 int ans[];

 void dfs(int step) {
     if (step == n) {
         int tmp = ans[] + ans[n - ];
         if (isPrime[tmp] == ) {
             printf("%d", ans[]);
             for (int j = ; j < n; j++) {
                 printf(" %d", ans[j]);
             }
             puts("");
         }
         return;
     }
     for (int i = ; i <= n; i++) {
         if (flag[i] == ) {
             if (step == ) {
                 flag[i] = ;
                 ans[step] = i;
                 dfs(step + );
                 flag[i] = ;
             }
             else {
                 int tmp = ans[step - ] + i;
                 if (isPrime[tmp] == ) {
                     flag[i] = ;
                     ans[step] = i;
                     dfs(step + );
                     flag[i] = ;
                 }
             }
         }
     }
 }
 int main(int argc, char const *argv[])
 {
     getPrime();
     memset(flag, , sizeof(flag));
     int p = ;
     while (scanf("%d", &n) != EOF) {
         printf("Case %d:\n", p);
         flag[] = ;
         ans[] = ;
         dfs();
         p++;
         puts("");
     }

     return ;
 }