input: Write a function to reverse the order of words in a given sentence while keeping the order of characters in each word unchanged. If the input is an empty string, return the empty string.
  assert reverse_words('This is a sentence.') == 'sentence. a is This'
  assert reverse_words('Hello World') == 'World Hello'
  assert reverse_words('Quick brown fox') == 'fox brown Quick'
  assert reverse_words('Single') == 'Single'
  assert reverse_words('') == ''
output: def reverse_words(sentence):
      words = sentence.split()
      reversed_sentence = ' '.join(words[::-1])
      return reversed_sentence