Prime Ring Problem hdu-1016 DFS

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. 

 

Inputn (0 < n < 20). 
OutputThe 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

给你一个数n  1-n组成一个环 相邻的两个数之和要是素数（所以同时要保证头和尾的和是素数）
DFS 基础题 
核心代码就一丢丢  很容易看懂

#include <iostream>
using namespace std;
#include<string.h>
#include<set>
#include<stdio.h>
#include<math.h>
#include<queue>
#include<map>
#include<algorithm>
#include<cstdio>
#include<cmath>
#include<cstring>
#include <cstdio>
#include <cstdlib>
#include<stack>
int n;
int a[25];
int b[25];
int sushu(int x)
{
    int s=(int)sqrt(x);
    for(int i=2;i<=s;i++)
    {
        if(x%i==0)
            return 0;
    }
    return 1;
}
void dfs(int x)
{
    if(x==n&&sushu(a[1]+a[n]))
    {
        int flag=0;
        for(int i=1;i<=n;i++)
        {
            if(!flag)
            {
                flag=1;
                printf("%d",a[i]);
            }
            else
            {
                printf(" %d",a[i]);
            }
        }
        printf("
");
        return;
    }
    for(int i=2;i<=n;i++)
    {
        if(b[i]==0&&sushu(a[x]+i))
        {
            //cout<<i<<endl;
            b[i]=1;
            a[x+1]=i;
            dfs(x+1);
            b[i]=0;
        }
    }
}
int main()
{
    int add=0;
    while(~scanf("%d",&n))
    {
        memset(a,0,sizeof(a));
        memset(b,0,sizeof(b));
        a[1]=1;
        printf("Case %d:
",++add);
        dfs(1);
        printf("
");
    }
    return 0;
}

讲道理 输入是奇数的时候直接跳过dfs就好 所以我加了个判断，结果时间还变长了一点 ，编程就像女人，永远搞不懂她。

贴上加判断的AC