input: Write a function to determine the maximum number in a list. If the list is empty, return None.
  def max_num(list):
      if not list:
          return None
      max_num = list[0]
      for num in list:
          if num > max_num:
              max_num = num
      return max_num
  
  assert max_num([9, 2, 5, 6, 4, 1])==9
  assert max_num([1, 2, 3, 4, 5])==5
  assert max_num([-10, -2, -5, -6, -4, -1])==-1
  assert max_num([])==None
output: def max_num(list):
      if not list:
          return None
      max_num = list[0]
      for num in list:
          if num > max_num:
              max_num = num
      return max_num