input: def reverse_words(string: str) -> str:
      """
      Reverse the order of words in a sentence while keeping the order of characters in each word unchanged.
      
      >>> reverse_words('This is a sentence.')
      'sentence. a is This'
      >>> reverse_words('Hello world')
      'world Hello'
      """
      words = string.split(" ")
      reversed_words = [words[i] for i in range(len(words) - 1, -1, -1)]
      return ' '.join(reversed_words)
output:     words = string.split(" ")
      reversed_words = [words[i] for i in range(len(words) - 1, -1, -1)]
      return ' '.join(reversed_words)