SortColorsCountingSort.java

public void sortColorsCountingSort(int[] A) {
    if (A == null || A.length == 0) return;

    int[] buckets = new int[3];

    for (int i = 0; i < A.length; i++) {
        buckets[A[i]] += 1;
    }

    int index = 0;
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < buckets[i]; j++) {
            A[index] = i;
            index++;
        }
    }
}