input: Write a function to find 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([])==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