牛客——将二叉搜索树转换成一个排序的双向链表

//将一个二叉搜索树转换成一个排序的双向链表，
//不能创建新节点，只能调整指针指向


这里会用到二叉搜索树的中序遍历，

#include<stack>
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)
    {
        typedef TreeNode* pNode;
        if (pRootOfTree == NULL)
            return NULL;
        stack<pNode> s1;
        pNode pCur = pRootOfTree;
        pNode pNext = NULL;
        pNode pPre = NULL;
        pNode pHead = NULL;
        while (pCur || !s1.empty())
        {
            while (pCur)
            {
                s1.push(pCur);
                pCur = pCur->left;
            }
            pCur = s1.top();
            pCur->left = pNext;
            if (pNext == NULL)
                pHead = pCur;
            pNext = pCur;
            s1.pop();

            pPre = pCur;
            pCur = pCur->right;
        }
        pPre->right = NULL;

        pNext = pPre;
        pCur = pNext->left;
        while (pCur)
        {
            pCur->right = pNext;
            pNext = pCur;
            pCur = pCur->left;
        }
        return pHead;
    }
};