input: Write a function to find the smallest two elements in a list.
  assert smallest_two_elements([45, 24, 2, 17, 78]) == (2, 17)
  assert smallest_two_elements([1, 2, 3, 4, 5]) == (1, 2)
  assert smallest_two_elements([10, 9, 8, 7, 6]) == (6, 7)
output: def smallest_two_elements(lst):
      if len(lst) < 2:
          return None
      first, second = float('inf'), float('inf')
      for num in lst:
          if num < first:
              first, second = num, first
          elif num < second:
              second = num
      return first, second