Java 8

public static List<Integer> countingSort(List<Integer> arr) {
// Write your code here

    // Integer maxInt = Collections.max(arr);

    int[] intArr = new int[100];

    for (Integer arg : arr) {
        intArr[arg] = intArr[arg] += 1;
    }

    return Arrays.stream(intArr).boxed().collect(Collectors.toList());
}