input: Write a function to determine the third largest element in a given list. If there are fewer than three distinct elements in the list, return None.
  assert third_largest([12, 13, 13, 45, 22, 99])==22
  assert third_largest([4, 3, 1, 2])==2
  assert third_largest([4, 3, 1, 2, 5])==3
  assert third_largest([3, 5, 4, 1, 2])==3
  assert third_largest([1, 2])==None
output: def third_largest(lst):
      unique_elements = list(set(lst))
      if len(unique_elements) < 3:
          return None
      unique_elements.sort(reverse=True)
      return unique_elements[2]