Monistus

import java.text.CharacterIterator;
import java.text.StringCharacterIterator;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.Queue;
import java.util.Scanner;

// Yes, this is a very odd solution.
public class Duplication {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        String inputString = input.nextLine();
        input.close();

        DuplicationIterator iterator = new DuplicationIterator(new CharIterator(inputString));
        for (char character = iterator.next(); iterator.hasNext(); character = iterator.next()) {
            int digit = Character.digit(character, 10);
            if (digit == -1) {
                System.out.print(character);
            } else {
                iterator.duplicate(digit);
                iterator = new DuplicationIterator(iterator);
            }
        }
        System.out.println(); // Final newline
    }

    public static class DuplicationIterator implements Iterator<Character> {
        private final Iterator<Character> delegate;
        private final Queue<Character> repeatQueue;
        private int copiedChars = 0;

        public DuplicationIterator(Iterator<Character> delegate) {
            this.delegate = delegate;
            this.repeatQueue = new LinkedList<>();
        }

        public void duplicate(int n) {
            copiedChars = n;
        }

        @Override
        public boolean hasNext() {
            return delegate.hasNext() || !repeatQueue.isEmpty();
        }

        /**
         * @return next character or CharacterIterator.DONE
         */
        @Override
        public Character next() {
            if (copiedChars == 0 && !repeatQueue.isEmpty()) {
                return repeatQueue.remove();
            }

            char next = delegate.next();
            if (copiedChars > 0) {
                copiedChars--;
                repeatQueue.add(next);
                if (next == CharacterIterator.DONE) {
                    System.out.println("!!!");
                }
            }
            return next;
        }
    }

    public static class CharIterator implements Iterator<Character> {
        private final CharacterIterator delegate;
        private boolean isAtBeginning = true;
        private boolean isAtEnd = false;

        public CharIterator(String string) {
            this.delegate = new StringCharacterIterator(string);
        }

        @Override
        public boolean hasNext() {
            return !isAtEnd;
        }

        @Override
        public Character next() {
            if (isAtBeginning) {
                isAtBeginning = false;
                return delegate.first();
            } else {
                char next = delegate.next();
                if (next == CharacterIterator.DONE) {
                    isAtEnd = true;
                }
                return next;
            }
        }
    }
}

Test 1

Group: 1, 2

Verdict: ACCEPTED

Test 2

Group: 1, 2

Verdict: ACCEPTED

Test 3

Group: 1, 2

Verdict: ACCEPTED

Test 4

Group: 1, 2

Verdict: ACCEPTED

Test 5

Group: 1, 2

Verdict: ACCEPTED

Test 6

Group: 1, 2

Verdict: ACCEPTED

Test 7

Group: 1, 2

Verdict: ACCEPTED

Test 8

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 9

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 10

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 11

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 12

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 13

Group: 2

Verdict: TIME LIMIT EXCEEDED

Test 14

Group: 2

Verdict: TIME LIMIT EXCEEDED