WordMap.scala

def makeWordMap(sentences: List[String]):
    Map[String, List[String]] = {

  val initMap = Map.empty[String, List[String]]

  sentences.foldLeft(initMap) { (map1, sentence) =>
    words(sentence).foldLeft(map1) { (map2, word) =>
      map2 +
        (word -> (sentence :: map2.getOrElse(word, Nil)))
    }
  }
}