二叉搜索树转化为有序双向链表

采用递归的方法实现，具体如下：

def Tree2List(root):
    if not root:
        return None, None
    if root.left:
        head, _tail = Tree2List(root.left)
        _tail.right = root
        root.left = _tail.right
    else:
        head = root
    if root.right:
        _head, tail = Tree2List(root.right)
        root.right = _head
        _head.left = root
    else:
        tail = root
    return head, tail