code: public void addPair(String first, String second)
      {
          Map<String, Integer> firstMatrix = transferMatrix.get(first);
          if (firstMatrix == null)
          {
              firstMatrix = new TreeMap<String, Integer>();
              transferMatrix.put(first, firstMatrix);
          }
          Integer frequency = firstMatrix.get(second);
          if (frequency == null) frequency = 0;
          firstMatrix.put(second, frequency + 1);
      }
docs: 添加一个转移例子，会在内部完成统计
  @param first
  @param second