code: public Collection<TermFrequency> top(int N)
      {
          MaxHeap<TermFrequency> heap = new MaxHeap<TermFrequency>(N, new Comparator<TermFrequency>()
          {
              @Override
              public int compare(TermFrequency o1, TermFrequency o2)
              {
                  return o1.compareTo(o2);
              }
          });
          heap.addAll(termFrequencyMap.values());
          return heap.toList();
      }
docs: 取前N个高频词
  
  @param N
  @return