StringBuilder result = new StringBuilder();
    PriorityQueue<Map.Entry<Character, Integer>> maxHeap =
            new PriorityQueue<>(((o1, o2) -> o2.getValue() - o1.getValue()));

    HashMap<Character, Integer> freqMap = new HashMap<>();

    for (char c : str.toCharArray()) {
        freqMap.merge(c, 1, Integer::sum);
    }

    maxHeap.addAll(freqMap.entrySet());

    while (!maxHeap.isEmpty()){
        Map.Entry<Character, Integer> entry = maxHeap.poll();
        String repeat = String.valueOf(entry.getKey()).repeat(entry.getValue());
        result.append(repeat);
    }

    // ToDo: Write Your Code Here.
    return result.toString();
}