input: Write a function to find the maximum number from a list of non-negative integers. If the given list is empty then return -1.
  def find_max(list):
      if not list:
          return -1
      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([1, 3, 5])==5
  assert find_max([0])==0
  assert find_max([])==-1
output: def find_max(list):
      if not list:
          return -1
      max_num = 0
      for i in list:
          if i > max_num:
              max_num = i
      return max_num