input: Write a python function to find the maximum number in a list.
  assert find_max([1, 2, 3, 4, 5]) == 5
  assert find_max([-1, -2, -3, -4]) == -1
  assert find_max([10, 20, 30, 40, 50]) == 50
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