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 = 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, 1, 2, 3, 4, 5]) == 5
  assert find_max([10, 10, 10]) == 10
  assert find_max([1]) == 1
  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