Pakkaus

import java.util.Scanner;

public class Pakkaus {

    public static void main(String[] args) {

        char[] compressed = new Scanner(System.in).nextLine().toCharArray();

        StringBuilder uncompressed = new StringBuilder();

        int i = 0;
        
        StringBuilder kStringBuilder = new StringBuilder();
        StringBuilder sStringBuilder = new StringBuilder();
        while (i < compressed.length) {
            kStringBuilder.setLength(0);
            while (Character.isDigit(compressed[i])) {
                kStringBuilder.append(compressed[i]);
                i++;
            }
            int k = Integer.parseInt(kStringBuilder.toString());

            sStringBuilder.setLength(0);
            while (i < compressed.length && !Character.isDigit(compressed[i])) {
                sStringBuilder.append(compressed[i]);
                i++;
            }
            
            for (int j = 0; j < k; j++) {
                uncompressed.append(sStringBuilder);
            }
        }

        System.out.println(uncompressed.toString());
    }

}

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