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 string is empty, return an empty string.
  
  def reverse_words(string):
      if not string:
          return ""
      words = string.split()
      reversed_words = words[::-1]
      return ' '.join(reversed_words)
  
  assert reverse_words('This is a sentence.') == 'sentence. a is This'
  assert reverse_words('Hello world') == 'world Hello'
  assert reverse_words('The quick brown fox jumps over the lazy dog') == 'dog lazy the over jumps fox brown quick The'
  assert reverse_words('') == ''
  assert reverse_words('Single') == 'Single'
output: def reverse_words(string):
      if not string:
          return ""
      words = string.split()
      reversed_words = words[::-1]
      return ' '.join(reversed_words)