input: Write a function to find the maximum number in a list and return it. If the list is empty then return None.
  assert find_max([1, 15, 20, 6]) == 20
  assert find_max([-5, -10, -7, -2]) == -2
  assert find_max([10]) == 10
  assert find_max([]) == None
output: def find_max(lst):
      if not lst:
          return None
      return max(lst)