input: Write a function to find the second highest number in a list.
  assert second_highest([12, 2, 6, 8, 19, 15]) == 15
  assert second_highest([5, 3, 9, 1, 7]) == 7
  assert second_highest([10, 20, 30, 40, 50]) == 40
output: def second_highest(numbers):
      if len(numbers) < 2:
          return None
      first = second = float('-inf')
      for number in numbers:
          if number > first:
              second = first
              first = number
          elif first > number > second:
              second = number
      return second