生成一棵以二叉链表存储的二叉树bt（不少于15个结点）。
2） 分别用递归和非递归方法前序遍历bt，并以缩格形式打印bt上各结点的信息。
3） 编写算法，交换bt上所有结点的左、右子树，并以缩格形式打印出交换前后的bt结点信息。(Generate a two fork tree BT (no less than 15 nodes) stored with a two chain list.
2) the BT is traversed with the preorder of recursive and non recursive methods, and the information of each node on BT is printed in the form of contraction.
3) write the algorithm, exchange the left and right subtrees of all nodes on the BT, and print out the BT node information before and after the exchange in the form of the contraction.)