HDOJ 1016 Prime Number

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.

#include <iostream>
int ring[20],vis[20];
using namespace std;
bool isPrimeNumber(int i){
   switch(i){
   case 2 :
   case 3 :
   case 5 :
   case 7 :
   case 11:
   case 13:
   case 17:
   case 19:
   case 23:
   case 29:
   case 31:
   case 37:
   case 39:return true;
    default:
       return false;
   }
}
void primeRing(int n,int i){
    if(i==n && isPrimeNumber(ring[0]+ring[n-1])){
      for(int j=0;j<n-1;j++)
        cout<<ring[j]<<'';
      cout<<ring[j]<<endl;
     }
     else{
        for(int j=2;j<=n;j++){  //create 1--n number
            if(!vis[j] && isPrimeNumber(ring[i-1]+j)){
               ring[i]=j;
               vis[j]=true;
               primeRing(n,i+1);
               vis[j]=false;
            }
        }//for
     }//else
}//primeRing
int main()
{
    memset(vis,false,sizeof(vis));
    memset(ring,0,sizeof(ring));
    int n,icase=0;
    ring[0]=1;
    vis[0]=true;  //Initialize
//    freopen("acm.txt","r",stdin);
    while(cin>>n && n>0 && n<20){
        icase++;
        cout<<"Case "<<icase<<":"<<endl;
        primeRing(n,1);
        cout<<endl;
    }
    return 0;
}

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