void PrintDuplicates(int[] arr)
   {
       if (arr == null)
           throw new ArgumentException("arr can't be null.");

       Hashtable table = new Hashtable();

       for (int i = 0; i < arr.Length; i++)
       {
           if (table.ContainsKey(arr[i]) == true)
           {
               // Console.WriteLine(i);
           }
           else
           {
               table.Add(arr[i], "");
           }
        }
    }

    void PrintDuplicatesWithSort(int[] arr)
    {
        if (arr == null)
            throw new ArgumentException("arr can't be null.");

        arr.ToList().Sort();

        for (int i = 0; i < arr.Length - 1; i++)
        {
            if (arr[i] == arr[i + 1])
            {
                // Console.WriteLine(i);
            }
        }
    }