問題描述：

輸入一棵二叉搜尋樹，將該二叉搜尋樹轉換成一個排序的雙向連結串列。要求不能建立任何新的結點，只能調整樹中結點指標的指向。

解題思路：

二叉搜尋樹的中序遍歷，同時每一次增加一個指向

# -*- coding:utf-8 -*-
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    def Convert(self, pRootOfTree):
        # write code here
        if not pRootOfTree:
            return None
        if not pRootOfTree.left and not pRootOfTree.right:
            return pRootOfTree
        left = self.Convert(pRootOfTree.left)
        if left:
            while left.right:
                left = left.right
            left.right = pRootOfTree
            pRootOfTree.left = left
        right = self.Convert(pRootOfTree.right)
        if right:
            while right.left:
                right = right.left
            right.left = pRootOfTree
            pRootOfTree.right = right
        while pRootOfTree.left:
            pRootOfTree = pRootOfTree.left 
        return pRootOfTree