Problem 35 Circular primes （STL：rotate）

Circular primes

Problem 35

The number, 197, is called a circular prime because all rotations of the digits: 197, 971, and 719, are themselves prime.

There are thirteen such primes below 100: 2, 3, 5, 7, 11, 13, 17, 31, 37, 71, 73, 79, and 97.

How many circular primes are there below one million?

代码：

#include<bits/stdc++.h>
using namespace std;

int isPrime(int n)
{  
  int k = sqrt(n);
  for(int i = 2; i <= k ; i++)
  {
    if(n % i == 0)
    {
      return 0;
    }
  }
  return 1;
}
int numbers[1000000] = {0};
int main()
{  
  int ans = 0;

  for(int i = 2; i < 1000000; i++)
  {
    numbers[i] = isPrime(i);
  }

  int isCircular;
  for(int i = 2; i < 1000000; i++)
  {

    isCircular = 1;
    stringstream ss;
    ss << i;
    string perm = ss.str();      

    for (int j = 0; j < perm.size(); j++)
    {      
      rotate(perm.begin(), perm.begin() + 1, perm.end());      
      if(!numbers[ atoi( perm.c_str() ) ])
      {
        isCircular = 0;
        break;
      }
   }
      if(isCircular)
        ans++;
  }
  cout<<ans<<endl;
  return 0;
}