public static List<int> countingSort(List<int> arr)
{
    var countMap = arr.GroupBy(n => n)
                    .OrderBy(g => g.Key)
                    .ToDictionary(g => g.Key, g => g.Count());

    List<int> zeros = Enumerable.Repeat(0, 100).ToList();

    for (int i = 0; i < zeros.Count; i++)
    {
        if (countMap.ContainsKey(i))
        {
            zeros[i] = countMap.GetValueOrDefault(i); 
        }
    }

    return zeros;
}