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([0, 0, 0, 0])==0
  assert find_max([4, 4, 4, 4])==4
  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