/**
 * 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* pre=NULL;
    TreeNode* head=NULL;
    TreeNode* convert(TreeNode* root) {
        if(!root) return root;
        dfs(root);
        return head;
    }
    void dfs(TreeNode* root){
        if(!root) return;
        dfs(root->left);
        if(pre==NULL) head=root;
        else pre->right=root;
        root->left=pre;
        pre=root;
        dfs(root->right);
    }
};