public static List<Integer> countingSort(List<Integer> arr) {
  int[] freqArray = new int[100];
  List<Integer> sortedList = new ArrayList<>();
  for (int num : arr) {
    freqArray[num]++;
  }

  for (int i = 0; i < freqArray.length; i++) {
    for (int j = 0; j < freqArray[i]; j++) {
      sortedList.add(i);
    }
  }

  return sortedList;
}