input: Write a function to reverse the order of words in a sentence but keep the order of characters in each word the same. If the input string is empty, return an empty string.
  assert reverse_words('This is a sentence.')=='sentence. a is This'
  assert reverse_words('Hello World')=='World Hello'
  assert reverse_words('Python is fun')=='fun is Python'
  assert reverse_words('Reverse this')=='this Reverse'
  assert reverse_words('')==''
output: def reverse_words(s):
      words = s.split()
      reversed_words = words[::-1]
      return ' '.join(reversed_words)