input: 
  def find_max(numbers: List[int]) -> int:
      """ Find the maximum number in a given list of integers.
      >>> find_max([9, 2, 5, 6, 4, 1])
      9
      >>> find_max([-1, -3, -5, -2, -4])
      -1
      """
  
output:     if not numbers:
          return None
      max_num = numbers[0]
      for num in numbers:
          if num > max_num:
              max_num = num
      return max_num