code: public int getFreq(int length, int loc) {
          if (locFreq == null)
              return 0;
          if (length > 3)
              length = 3;
          if (loc > 4)
              loc = 4;
          return locFreq[length][loc];
      }
docs: 得道某一个位置的词频
  
  @param length
  @param loc
  @return