input: Write a function to return the maximum number in a list. The function should iterate through the list and keep track of the highest number found. If the list is empty, it should return None.
  assert find_max([1, 15, 20, 6]) == 20
  assert find_max([10, 5, 30, 25]) == 30
  assert find_max([-5, -10, -2, -1]) == -1
  assert find_max([]) == None
  assert find_max([42]) == 42
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