input: Write a function to return the maximum number in a list. If the list is empty, return None.
  
  assert find_max([1, 15, 20, 6])==20
  assert find_max([3, 5, 4, 1, 2])==5
  assert find_max([-10, -20, -30, -5])==-5
  assert find_max([7])==7
  assert find_max([])==None
output: def find_max(lst):
      if not lst:
          return None
      return max(lst)