HDU 1241 Prime Ring Problem

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#define maxn 25
#define inf 0x3f3f3f3f
using namespace std;
int n;
int a[maxn];
bool vis[maxn];
bool prime(int n){
    for(int i=2;i*i<=n;i++){
        if(n%i==0)return false;
    }
    return n!=1;
}
void dfs(int cur){
    if(cur>n){
        if(prime(a[cur-1]+1)){    //他是一个环，最后要记得判断最后一个和第一个和是否为质数
        for(int i=1;i<=n;i++){
           if(i!=n)printf("%d ",a[i]);
           else printf("%d\n",a[i]);
        }
        }
        return ;
    }
    for(int i=2;i<=n;i++){
            if(prime(a[cur-1]+i)&&vis[i]==false){
                vis[i]=true;
                a[cur]=i;
                dfs(cur+1);
                vis[i]=false;
            }
    }
}
int main()
{
    int i=1;
    //freopen("in.txt","r",stdin);
    while(scanf("%d",&n)!=EOF&&n){
            memset(vis,0,sizeof vis);
        printf("%s %d%s\n","Case",i++,":");
        vis[1]=true;
        a[1]=1;
        dfs(2);
        puts("");
    }
}

<div class="hiddable" id="vj_description" style="font-family: 'times new roman'; font-size: 17px; color: rgb(34, 34, 34);"><p class="pst" style="font-family: Arial, Helvetica, sans-serif; font-size: 18pt; font-weight: bold; color: blue; margin-bottom: 0px;">Description</p><div class="textBG" style="background-color: rgb(234, 235, 255); border-top-left-radius: 10px; border-top-right-radius: 10px; border-bottom-right-radius: 10px; border-bottom-left-radius: 10px; padding: 10px; border-style: dotted; border-width: 2px;"><div class="panel_content">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. 

<img src="http://acm.hdu.edu.cn/data/images/1016-1.gif" alt="" />
</div><div class="panel_bottom"> </div></div></div><div class="hiddable" id="vj_input" style="font-family: 'times new roman'; font-size: 17px; color: rgb(34, 34, 34);"><p class="pst" style="font-family: Arial, Helvetica, sans-serif; font-size: 18pt; font-weight: bold; color: blue; margin-bottom: 0px;">Input</p><div class="textBG" style="background-color: rgb(234, 235, 255); border-top-left-radius: 10px; border-top-right-radius: 10px; border-bottom-right-radius: 10px; border-bottom-left-radius: 10px; padding: 10px; border-style: dotted; border-width: 2px;"><div class="panel_content">n (0 < n < 20). 
</div><div class="panel_bottom"> </div></div></div><div class="hiddable" id="vj_output" style="font-family: 'times new roman'; font-size: 17px; color: rgb(34, 34, 34);"><p class="pst" style="font-family: Arial, Helvetica, sans-serif; font-size: 18pt; font-weight: bold; color: blue; margin-bottom: 0px;">Output</p><div class="textBG" style="background-color: rgb(234, 235, 255); border-top-left-radius: 10px; border-top-right-radius: 10px; border-bottom-right-radius: 10px; border-bottom-left-radius: 10px; padding: 10px; border-style: dotted; border-width: 2px;"><div class="panel_content">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. 
</div><div class="panel_bottom"> </div></div></div><div class="hiddable" id="vj_sampleInput" style="font-family: 'times new roman'; font-size: 17px; color: rgb(34, 34, 34);"><p class="pst" style="font-family: Arial, Helvetica, sans-serif; font-size: 18pt; font-weight: bold; color: blue; margin-bottom: 0px;">Sample Input</p><div class="textBG" style="background-color: rgb(234, 235, 255); border-top-left-radius: 10px; border-top-right-radius: 10px; border-bottom-right-radius: 10px; border-bottom-left-radius: 10px; padding: 10px; border-style: dotted; border-width: 2px;"><div class="panel_content"><pre style="white-space: pre-wrap; word-wrap: break-word;"><div style="font-family: 'Courier New', Courier, monospace;"> 6
8 </div>

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