public String frequencySort(String s) {
    // use array to save the char and it's frequency
    int[] charToFre = new int[256];
    for (int i = 0; i < s.length(); i++) {
        char c = s.charAt(i);
        charToFre[c]++;
    }

    PriorityQueue<Integer> sortResult = new PriorityQueue<>
        ((a, b) -> (charToFre[a] == charToFre[b] ? a - b : charToFre[b] - charToFre[a]));

    for (int i = 0; i < 256; i++) {
        if (charToFre[i] > 0) {
            sortResult.add(i);
        }
    }
    
    // the int should be convert to char!!!
    StringBuilder result = new StringBuilder();
    while (!sortResult.isEmpty()) {
        int i = sortResult.poll();
        for (int j = 0; j < charToFre[i]; j++) {
            result.append((char)(i));
        }
    }
    return result.toString();
}