St.pop() Error unexpected

#1

import java.util.*;

public class BalanceBrackets{

public static String isBalanced(String s){

     boolean res = false;
    int len = s.length();

    // Stack<Character> st = new Stack<>(); 
     Stack st = new Stack();

    for(int i=0;i<=len;i++){
            char ch = s.charAt(i);

        if(ch =='['|| ch=='{' || ch=='(' ){
           st.push(ch);
        }

        if(ch ==']'|| ch=='}' || ch==')'){
            char c = st.pop();
            res = check(c,ch);
           
            }

        }
        String a = "NO";
        String b = "YES";

        if(!s.isEmpty()){
            return a;
        }else{
            if(res== true)
            return b;
        }
        System.out.println();
        return a;
    }




public static boolean check (char character1,char character2){

     if (character1 == '(' && character2 == ')') 
     return true; 
   else if (character1 == '{' && character2 == '}') 
     return true; 
   else if (character1 == '[' && character2 == ']') 
     return true; 
   else
     return false; 
} 
 public static void main(String[] args) {

    String str = "[{()}]";
    String result = isBalanced(str);
    System.out.println(result);
     
 }
 }
#2

Before making a pop from the stack, you are not checking if the stack is having elements or not. So, in case it is empty it will throw exception

#3

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