[코테2_6] 뒤집은 소수

import java.util.*;

public class P2_6{
 public static ArrayList<Integer> solution(ArrayList<String> list){
   ArrayList<Integer> list_n = new ArrayList<Integer>();
   for(String x: list){
         StringBuffer sb = new StringBuffer(x);
         String str_= sb.reverse().toString();
         int str_n =Integer.parseInt(str_);
         for(int i=2;i<str_n;i++){
               if(str_n%i==0)
                     break;
               if(i==str_n-1){
                     list_n.add(str_n);
                  }
          }
       }
    return list_n;
  }
 public static void main(String[] args){
  Scanner kb = new Scanner(System.in);
  int N = kb.nextInt();
  ArrayList<String> list = new ArrayList<String>();
  for(int i=0;i<N;i++){
    String input = kb.next();
    list.add(input);
   }
  for(int x : solution(list))
     System.out.print(x+" "); 

  }

 }

뭔가 이상함

강의 듣고 힌트 얻어서 다시 푼 결과 -> 성공

import java.util.*;

public class P2_6{
  public static boolean isPrime(int n){
     if (n==1) return true;
     for(int i=2;i<n;i++){
            if(n%i==0) return true;
       }
       return false;
   }

  public static ArrayList<Integer> solution(int n, ArrayList<Integer> list){
        ArrayList<Integer> answer = new ArrayList<>();
         for(int i=0;i<n;i++){
           int tmp=list.get(i);
           int t=0, res=0;
           while(tmp!=0){
                   t=tmp%10;
                   res=res*10+t;
                   tmp=tmp/10;
           } //while
           if(!isPrime(res)) answer.add(res);
         }//for
      return answer;
   }

  public static void main(String[] args){
   Scanner kb = new Scanner(System.in);
   int n = kb. nextInt();
   ArrayList<Integer> list =new ArrayList<>();
   for(int i=0;i<n;i++){
            int input = kb.nextInt();
            list.add(input);
      }
    for(int x : solution(n,list)) System.out.print(x+" ");

  }
}