input: Write a function to find the maximum number from a list of non-negative integers. If the list is empty, return -1.
  def find_max(list):
      if not list:
          return -1
      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, 7, 3, 9]) == 9
  assert find_max([]) == -1
  assert find_max([1]) == 1
output: def find_max(list):
      if not list:
          return -1
      max_num = list[0]
      for i in list:
          if i > max_num:
              max_num = i
      return max_num