public List<String> popularNFeatures(int numFeatures, int topFeatures, List<String> possibleFeatures,
                                              int numFeatureRequests, List<String> featureRequests) {
        List<String> res = new ArrayList<>();
        Set<String> features = new HashSet(possibleFeatures);
        Map<String, Word> wordsMap = new HashMap<>();

        for(int i = 0; i < numFeatureRequests; i++) {
            String featureRequest = featureRequests.get(i).replaceAll("[\\!?,;.]", "").toLowerCase();
            String[] words = featureRequest.split(" ");

            for(int j = 0; j < words.length; j++) {
                String word = words[j];
                if(features.contains(word)) {
                    Word w;

                    if(wordsMap.containsKey(word))
                        w = wordsMap.get(word);
                    else
                        w = new Word(word, 0);
                    w.count++;
                    w.featureRequestIds.add(i);
                    wordsMap.put(word, w);
                }
            }
        }

        PriorityQueue<Word> pq = new PriorityQueue<>(new Comparator<Word>() {

            @Override
            public int compare(Word w1, Word w2) {
                if(w1.count != w2.count)
                    return Integer.compare(w2.count, w1.count);
                else if(w1.featureRequestIds.size() != w2.featureRequestIds.size())
                    return Integer.compare(w2.featureRequestIds.size(), w1.featureRequestIds.size());
                else
                    return w1.word.compareTo(w2.word);
            }
        });

        pq.addAll(wordsMap.values());

        if(topFeatures > pq.size()) {
            for (int i = 0; i < numFeatures && !pq.isEmpty(); i++)
                res.add(pq.poll().word);
        } else {
            for(int i = 0; i < pq.size(); i++) {
                res.add(pq.poll().word);
            }
        }

        return res;
    }

    private static class Word {
        String word;
        int count;
        Set<Integer> featureRequestIds;
        public Word(String word, int count) {
            this.word = word;
            this.count = count;
            this.featureRequestIds = new HashSet<>();
        }
    }