素数环问题

#include <iostream>
#include <stdlib.h>
#include <math.h>
using namespace std;
#include <stdio.h>
#include <math.h>
#define LEN 20
int primeRing(int ring[], int b[], int n);
int isPrime(int n);
int main(void)
{
 int i, ring[LEN]={0}, b[LEN]={0};
 ring[0] = 1;
 b[0] = 1;
 if( primeRing(ring, b, 1) )
 {
  //printf("\n\nThe prime ring is : ");
  for(i=0; i<LEN; i++)
   printf("%d ", ring[i]);
  //printf("\n");
 }
 else
  printf("\n\nNot found!\n");
 return 0;
}

int primeRing(int ring[], int b[], int n)
{
 int i;
 if( n==LEN )
  return isPrime(ring[n-1]+ring[0]);
 //printf("\nCalculating ring[%d] = ", n);
 for(i=1; i<LEN; i++)
  if( b[i]==0 && isPrime((i+1)+ring[n-1]) )
  {
   b[i] = 1;
   //cout<<"  "<<i+1<<endl;
   ring[n] = i+1;
   //printf("%d ", ring[n]);
   if( primeRing(ring, b, n+1) )
    return 1;
   else
    b[i] = 0;
  }
 return 0;
}

int isPrime(int n)
{
 int i, t, f=1;
 t = sqrt(n);
 for(i=2; i<=t; i++)
  if( n%i==0 )
  {
   f = 0;
   break;
  }
 return f;
}