소수 찾기

https://school.programmers.co.kr/learn/courses/30/lessons/42839

https://mjmjmj98.tistory.com/50

import java.util.*;

class Solution {
    public static int[] chk = new int[7];
    public static Set<Integer> set = new HashSet<>();
    
    public static boolean chkNumber(int num)
    {
        if(num <= 1) return false;

        for(int i=2; i<= (int)Math.sqrt(num); i++)
        {
            if(num%i == 0)
            {
                return false;
            }
        }
        return true;
    }
    
    public static void dfs(String word, String makeWord, int depth)
    {
        if(makeWord.length() == depth)
        {
            set.add(Integer.parseInt(makeWord));
        }

        for(int i=0; i<word.length(); i++)
        {
            if(chk[i] == 0)
            {
                chk[i] = 1;
                dfs(word, makeWord + word.charAt(i), depth);
                chk[i] = 0;
            }
        }
    }
    
    public static int solution(String word) {
        int answer = 0;

        for(int i=0; i<word.length(); i++)
        {
            dfs(word,"",i+1);
        }

        for(var num : set) {
            if (chkNumber(num))
            {
                answer++;
            }
        }

        return answer;
    }
}

익힐것
1. Set set = new HashSet<>();
2.