input: Write a python function to find the maximum number in a list.
  assert find_max([1, 3, 12, 5, 8, 10]) == 12
  assert find_max([5, 7, 2, 8, 3, 10]) == 10
  assert find_max([-5, -3, -12, -1, -8]) == -1
output: def find_max(lst):
      if not lst:
          return None
      max_num = lst[0]
      for num in lst:
          if num > max_num:
              max_num = num
      return max_num