//参考 https://cuijiahua.com/blog/2017/12/basis_26.html
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* convert(TreeNode* root) {
        TreeNode* pLast=NULL;
        ConvertNode(root,&pLast);

        TreeNode* pHead = pLast;
        while(pHead!=NULL && pHead->left!=NULL){
            pHead = pHead->left;
        }
        return pHead;
    }
    void ConvertNode(TreeNode* pNode,TreeNode** pLast){
        if(pNode==NULL) return;

        if(pNode->left!=NULL){
            ConvertNode(pNode->left,pLast);
        }
        //连接左右指针
        pNode->left = (*pLast);
        if((*pLast)!=NULL){
            (*pLast)->right = pNode;
        }

        //更新尾节点
        *pLast = pNode;
        if(pNode->right!=NULL){
            ConvertNode(pNode->right,pLast);
        }
    }
};