题目描述：

输入一棵二叉搜索树，将该二叉搜索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，只能调整树中结点指针的指向。

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {
 }
};*/
class Solution {
public:
    TreeNode* Convert(TreeNode* pRootOfTree)
    {
        TreeNode *pLastNodeOfList = NULL;
        ConvertNode(pRootOfTree, &pLastNodeOfList);

        TreeNode *pHeadOfList = pLastNodeOfList;
        while(pHeadOfList!=NULL && pHeadOfList->left!=NULL)
            pHeadOfList = pHeadOfList->left;
        return pHeadOfList;
    }
private:
    void ConvertNode(TreeNode *pNode, TreeNode **pLastNode){
        if(!pNode) return;
        TreeNode *pCurrent = pNode;
        if(pCurrent->left)
            ConvertNode(pCurrent->left, pLastNode);
        pCurrent->left = *pLastNode;
        if(*pLastNode)
            (*pLastNode)->right = pCurrent;
        *pLastNode = pCurrent;
        if(pCurrent->right)
            ConvertNode(pCurrent->right, pLastNode);
    }
};

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