Monistus

import java.util.*;

class Monistus {
    public static String repeat(int count, String str){
        return new String(new char[count]).replace("\0", str);
    }
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String input = sc.nextLine();

        String start = "";
        int i = 0;
        while(i < input.length()){
            Character character = input.charAt(i);
            
            if(Character.isDigit(character)){
                int digit = Character.getNumericValue(character);
                input = start + repeat(2, input.substring(i+1, i+digit+1)) + input.substring(i+digit+1, input.length());
                i -= 1;
            }
            else{
                start += input.charAt(i);
            }
            i += 1;
        }
        System.out.println(input);
    }
}

Test 1

Group: 1, 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 2

Group: 1, 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 3

Group: 1, 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 4

Group: 1, 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 5

Group: 1, 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 6

Group: 1, 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 7

Group: 1, 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 8

Group: 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 9

Group: 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 10

Group: 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 11

Group: 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 12

Group: 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 13

Group: 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...

Test 14

Group: 2

Verdict: RUNTIME ERROR

Error: Could not find or load main class monistus
Caused by: java.lang.ClassNotFoundExcept...