说明：  可以实现 二叉树的先序遍历、中序遍历、后序遍历、层次遍历、二叉树的高度结点数计算、叶子结点数计算、左右子树交换后的结果
(It can realize the first order traversal, the middle order traversal, the second order traversal, the level traversal, the height node number calculation, the leaf node number calculation, the result after the left and right subtrees exchange)