input: Write a function to find the maximum number from a list of non-negative integers. If the list is empty, return None.
  def find_max(list):
      if not list:
          return None
      max_num = 0
      for i in list:
          if i > max_num:
              max_num = i
      return max_num
  
  assert find_max([1, 3, 12, 5, 8, 10])==12
  assert find_max([23, 45, 67, 12, 8])==67
  assert find_max([1])==1
  assert find_max([0, 0, 0, 0])==0
  assert find_max([])==None
output: def find_max(list):
      if not list:
          return None
      max_num = 0
      for i in list:
          if i > max_num:
              max_num = i
      return max_num