input: def reverse_words(string: str) -> str:
      """
      Reverse the order of words in a given 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 = []
      for word in words:
          reversed_words.insert(0, word)
      return ' '.join(reversed_words)
output:     words = string.split(" ")
      reversed_words = []
      for word in words:
          reversed_words.insert(0, word)
      return ' '.join(reversed_words)