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