백준 1252번 반례찾기

import java.util.*;


class Main{
 public static StringBuilder solution(String s1,String s2){
     Stack <Integer> st = new Stack<>();
     int mok =0;
     int s1p = s1.length()-1;
     int s2p = s2.length()-1;
     int result =0;
     while(s1p>=0 || s2p>=0){
           result=0;
           if(s1p<0) result = (s2.charAt(s2p--)-'0') + mok;
           else if(s2p<0)  result = (s1.charAt(s1p--)-'0') + mok;
           else result = (s1.charAt(s1p--)-'0') + (s2.charAt(s2p--)-'0') + mok;
           if(result==3){
              mok=1;
              st.push(1);
              } 
            else if(result==2){
              mok=1;
              st.push(0);
              }
            else {
              mok=0;
              st.push(result);
              }
            if(s1p==-1 &&s2p==-1&&mok==1) st.push(mok);
      }
      StringBuilder sb = new StringBuilder();
      while(st.size()>0){
          sb.append(st.pop());
       }
      
     return sb;
    
  
 } 
 public static void main(String[] args){
    Scanner kb = new Scanner(System.in);
    String s1 = kb.next();
    String s2 = kb.next();
 
    System.out.println(solution(s1,s2));
  }
}