extended binary tree

• 扩充的二叉树,扩展的二叉树

extended binary tree的用法和样例：

例句

1. Write a function to find the depth of a binary tree.
   写个程序找到一个二叉树的深度。
2. If a binary tree becomes degenerate, we run into the same efficiency problems as we did with the singly linked list.
   如果二叉树变得很不平衡，我们将会碰到同样的效率问题如同在持久化单向链表是一样。