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([-1, -5, -3])
      -1
      >>> max_num([0, 0, 0])
      0
      >>> max_num([10, 100, 1000])
      1000
      """
output:     if not list:
          return None
      max_val = list[0]
      for num in list:
          if num > max_val:
              max_val = num
      return max_val