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 = list[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
  assert find_max([])==None
output: def find_max(list): 
    if not list:
      return None
    max_num = list[0]
    for i in list: 
      if i > max_num: 
        max_num = i
    
    return max_num