chapter 10 10.4-5 要求不用Stack来实现对树的遍历，关键在树stack的作用是记录parent节点的信息，故给每个node 添加parent的属性就可以解决这个问题。以root为当前node，prev记录上一个root, 比较prev和root的关系 决定是Left，Right 访问或者回溯到parent。
10.4-6 去掉parent指针，增加一个bool确定node有没有right sibling，有的话next指向兄弟，没有的话next指向parent。
chapter 12 12.2 Successor and Precessor, which is O(h) , h is the height of the tree for a continuous (n-1) Successor call, it at most visit every edge 2 times, which time complexcity is O(n).
*Red-black tree insertion and deletion. https://github.com/julycoding/The-Art-Of-Programming-By-July/blob/master/ebook/zh/03.01.md