input: from typing import List
  
  def max_num(list: List[int]) -> int:
      """
      Find the maximum number in a given list.
      >>> max_num([9, 2, 5, 6, 4, 1])
      9
      >>> max_num([10, 5, 7, 12, 3])
      12
      """
output:     if not list:
          return None
      max_value = list[0]
      for num in list:
          if num > max_value:
              max_value = num
      return max_value