input: def third_largest(lst):
      """
      Get the third largest element in a given list.
      >>> third_largest([12, 13, 13, 45, 22, 99])
      22
      >>> third_largest([1, 3, 2, 4, 5, 6, 6, 5, 4])
      4
      """
output:     lst = sorted(set(lst), reverse=True)
      if len(lst) < 3:
          return None
      return lst[2]