> /**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    //不能放执行的句子  如t=res
    TreeNode *t=new TreeNode(-1);
    TreeNode *res=t;
    TreeNode* increasingBST(TreeNode* root) {
         if(root==nullptr)
            return res->right;

         increasingBST(root->left);
         t->right=root;
         root->left=nullptr;//root左儿子全被调用了
         t=root;
        increasingBST(root->right);
        return res->right;
    }

    // TreeNode* t;//临时  放外边
    // TreeNode* increasingBST(TreeNode* root) {
    //     TreeNode* res=new TreeNode(-1);
    //     t=res;
    //     dfs(root);
    //     return res->right;
    // }
    // void dfs(TreeNode* root){
    //     if(!root)
    //     return;
    //     dfs(root->left);
    //     t->right=root;
    //     root->left=NULL;//将root左子树设为空  
    //     t=root;// t更新
    //     dfs(root->right);
    // }
};