code: public List<String> extractSuffix(int length, int size, boolean extend)
      {
          TFDictionary suffixTreeSet = new TFDictionary();
          for (String key : tfDictionary.keySet())
          {
              if (key.length() > length)
              {
                  suffixTreeSet.add(key.substring(key.length() - length, key.length()));
                  if (extend)
                  {
                      for (int l = 1; l < length; ++l)
                      {
                          suffixTreeSet.add(key.substring(key.length() - l, key.length()));
                      }
                  }
              }
          }
  
          if (extend)
          {
              size *= length;
          }
  
          return extract(suffixTreeSet, size);
      }
docs: 提取公共后缀
  @param length 公共后缀长度
  @param size 频率最高的前多少个公共后缀
  @param extend 长度是否拓展为从1到length为止的后缀
  @return 公共后缀列表