input: Write a python function to find the second highest number in a list.
  assert second_highest_number([8, 5, 11, 6, 9, 15]) == 11
  assert second_highest_number([1, 3, 4, 7, 2, 9]) == 7
  assert second_highest_number([10, 20, 30, 40, 50]) == 40
output: def second_highest_number(lst):
      if len(lst) < 2:
          return None
      first, second = float('-inf'), float('-inf')
      for number in lst:
          if number > first:
              first, second = number, first
          elif first > number > second:
              second = number
      return second