input: def third_largest(lst):
      """Returns the third largest element in a given list of numbers. If there are less than 3 distinct numbers in the list, return None.
      Example:
      >>> third_largest([12, 13, 13, 45, 22, 99])
      22
      >>> third_largest([10, 20])
      None
      """
output:     if len(lst) < 3:
          return None
      unique_numbers = list(set(lst))
      unique_numbers.sort(reverse=True)
      if len(unique_numbers) < 3:
          return None
      return unique_numbers[2]