输入一棵二分查找树，将该二元查找树转换成一个排序的双向链表。

要求不能创建任何新的结点，只调整指针的指向。

二分查找树：root，每个节点：data、small pointer、large pointer

转换成排序的双向链表后small pointer应该指向前一个元素，larger pointer指向后一个元素-Enter a binary search tree, the two yuan search tree into a sort of two-way linked list. Requirements can not create any new node, adjust the pointers point only. Binary search tree: root, each node: data, small pointer, large pointer after converted into a doubly linked list before ordering small pointer should point to an element, after the larger pointer points to an element