TreeNode* convert(TreeNode* root) {
        TreeNode* head = nullptr, *cur = nullptr;
        function<void(TreeNode*)> _convert = [&](TreeNode* root){
            if (root == nullptr) return;
            if (root->left != nullptr)
                _convert(root->left);
            else {
                if (head == nullptr) head = root;
            }
            if (cur != nullptr) {
                cur->right = root;
                root->left = cur;
            } 
            cur = root;
            _convert(root->right);
        };
        _convert(root);
        return head;
    }