프로그래머스 소수찾기

import java.util.HashSet;
class Solution {
    public int solution(String numbers) {
        HashSet<Integer> set = new HashSet<>();
        permutation("", numbers, set);
        int count = 0;
        while(set.iterator().hasNext()){
            int a = set.iterator().next();
            set.remove(a);
            if(a==2) count++;
            if(a%2!=0 && isPrime(a)){
                count++;
            }
        }        
        return count;
    }

    public boolean isPrime(int n){
        if(n==0 || n==1) return false;
        for(int i=3; i<=(int)Math.sqrt(n); i+=2){
            if(n%i==0) return false;
        }
        return true;
    }

    public void permutation(String prefix, String str, HashSet<Integer> set) {
        int n = str.length();
        //if (n == 0) System.out.println(prefix);
        if(!prefix.equals("")) set.add(Integer.valueOf(prefix));
        for (int i = 0; i < n; i++)
          permutation(prefix + str.charAt(i), str.substring(0, i) + str.substring(i+1, n), set);

    }

}