input: Write a function to find the maximum number in a given 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([3, 5, 10, 1, 2])==10
  assert max_num([4, 3, 1, 2])==4
  assert max_num([])==None
  assert max_num([7])==7
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