input: Write a function to find the maximum number in a list. Return the maximum number. If the list is empty, return None.
  
  assert max_num([9, 2, 5, 6, 4, 1])==9
  assert max_num([0, 0, 0, 0])==0
  assert max_num([-5, -3, -9, -2])==-2
  assert max_num([])==None
output: def max_num(lst):
      if not lst:
          return None
      return max(lst)