Pakkaus

import java.lang.reflect.Field;
import java.util.Scanner;

public class Pakkaus {

    public static void main(String[] args) throws IllegalArgumentException, IllegalAccessException, NoSuchFieldException {
        StringBuilder mj = new StringBuilder();
        StringBuilder lj = new StringBuilder();
        StringBuilder output = new StringBuilder();
        boolean num = true;
        Scanner lukija = new Scanner(System.in);
        String input = lukija.nextLine();
        char merkki;
        final Field field = String.class.getDeclaredField("value");
        field.setAccessible(true);

        final char[] chars = (char[]) field.get(input);
        final int pituus = chars.length;

        for (int i = 0; i < pituus; i++) {
            merkki = input.charAt(i);

            if (Character.isDigit(merkki)) {
                if (num == false) {
                    num = true;
                    output.append(new String(new char[Integer.parseInt(lj.toString())]).replace("\0", mj));
                    lj.setLength(0);
                    mj.setLength(0);

                }
                lj.append(merkki);

            } else {
                if (num == true) {
                    num = false;
                }
                mj.append(merkki);
            }
        }
        
        output.append(new String(new char[Integer.parseInt(lj.toString())]).replace("\0", mj));
        System.out.println(output);
        System.out.println("");
    }
}

Test 1

Group: 1

Verdict: ACCEPTED

Test 2

Group: 1

Verdict: ACCEPTED

Test 3

Group: 1

Verdict: ACCEPTED

Test 4

Group: 1

Verdict: ACCEPTED

Test 5

Group: 1

Verdict: ACCEPTED

Test 6

Group: 2

Verdict: ACCEPTED

Test 7

Group: 2

Verdict: ACCEPTED

Test 8

Group: 2

Verdict: ACCEPTED

Test 9

Group: 2

Verdict: ACCEPTED

Test 10

Group: 2

Verdict: ACCEPTED

Test 11

Group: 3

Verdict: ACCEPTED

Test 12

Group: 3

Verdict: ACCEPTED

Test 13

Group: 3

Verdict: ACCEPTED

Test 14

Group: 3

Verdict: ACCEPTED

Test 15

Group: 3

Verdict: ACCEPTED