input: Write a function to return the maximum number in a given list. The function should return the largest number found in the list. If the list is empty, the function should return None.
  assert find_max([1, 15, 20, 6])==20
  assert find_max([100, 30, 50, 40])==100
  assert find_max([-1, -5, -3, -10])==-1
  assert find_max([42])==42
  assert find_max([])==None
output: def find_max(lst):
      if not lst:
          return None
      return max(lst)