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