input: Write a function to reverse the order of words in a given sentence while keeping the order of characters in each word unchanged.
  assert reverse_words('This is a sentence.')=='sentence. a is This'
  assert reverse_words('Hello world')=='world Hello'
  assert reverse_words('Python is awesome')=='awesome is Python'
  assert reverse_words('Single')=='Single'
  assert reverse_words('')==''
output: def reverse_words(s):
      words = s.split()
      reversed_words = words[::-1]
      return ' '.join(reversed_words)