code: public int[] getTopSentence(int size)
      {
          Collection<Integer> values = top.values();
          size = Math.min(size, values.size());
          int[] indexArray = new int[size];
          Iterator<Integer> it = values.iterator();
          for (int i = 0; i < size; ++i)
          {
              indexArray[i] = it.next();
          }
          return indexArray;
      }
docs: 获取前几个关键句子
  
  @param size 要几个
  @return 关键句子的下标