WordMap.java

public Map<String, List<String>> makeWordMap(
    List<String> sentences) {

  Map<String, List<String>> result =
    new HashMap<String, List<String>>();

  for (String sentence: sentences) {
    for (String word: words(sentence)) {
      List<String> sentencesForWord = result.get(word);
      if (sentencesForWord == null) {
        sentencesForWord = new ArrayList<String>();
        result.put(word, sentencesForWord);
      }
      sentencesForWord.add(sentence);
    }
  }

  return result;
}