input: Write a function to reverse the order of words in a given sentence while keeping the order of characters in each word unchanged. The function should return the modified sentence. If the input string is empty, the function should 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('Python is fun')=='fun is Python'
  assert reverse_words('')==''
  assert reverse_words('OneWord')=='OneWord'
output: def reverse_words(string):
      if not string:
          return ''
      words = string.split()
      reversed_words = words[::-1]
      return ' '.join(reversed_words)