input: Write a function that returns the third largest element in a given list. If the list has fewer than three elements, return None.
  assert third_largest([12, 13, 13, 45, 22, 99])==22
  assert third_largest([3, 2, 1])==1
  assert third_largest([1, 2])==None
  assert third_largest([5, 4, 3, 2, 1, 2, 3, 4, 5])==4
  assert third_largest([])==None
output: def third_largest(lst):
      if len(lst) < 3:
          return None
      unique_sorted = sorted(set(lst), reverse=True)
      if len(unique_sorted) < 3:
          return None
      return unique_sorted[2]