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 great') == 'great is Python'
  assert reverse_words('') == ''
output: def reverse_words(sentence):
      words = sentence.split()
      reversed_words = words[::-1]
      return ' '.join(reversed_words)