说明：  采用python实现红黑树的算法，整体思路与《算法入门》中的红黑树构造相似，代码符合红黑树的五大特征，是中大考试题的参考答案改编而来
(Using Python to realize the algorithm of red black tree, the overall idea is similar to the structure of red black tree in "Introduction to algorithm". The code conforms to the five characteristics of red black tree, and is adapted from the reference answer of CUHK examination questions)