创建二叉树类。二叉树的存储结构使用链表。

提供操作:前序遍历、中序遍历、后序遍历、层次遍历、计算二叉树结点数目、计算二叉树高度。

-Create a binary tree class. Binary tree using linked list storage structure. Provides operations: pre-order traversal, inorder traversal, postorder traversal, traverse levels to calculate the number of binary tree nodes to calculate binary tree height.