为什么更改树的PreOrder遍历的实现是错误的？

如何解决为什么更改树的PreOrder遍历的实现是错误的？

请告诉我为什么在遍历树的PreOrder方法2中是错误的。我应该使用哪种方式？那么主要的区别是TreeNode *是method1的返回值，而TreeNode *root是method2的参数。

method1: right
    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        istringstream in(data);
        return deserialize(in);
    }
    TreeNode *deserialize(istringstream &in)
    {
        string val;
        in >> val;
        if (val == "#") {
            return nullptr;
        }
        TreeNode *root = new TreeNode(stoi(val));
        root->left = deserialize(in);
        root->right = deserialize(in);
        return root;
    }

method2: wrong
    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        istringstream in(data);
        TreeNode *res;
        deserialize(in,res);
        return res;
    }
    void deserialize(istringstream &in,TreeNode *root)
    {
        string val;
        in >> val;
        if (val == "#") {
            root = nullptr;
            return;
        }
        root = new TreeNode(stoi(val));
        deserialize(in,root->left);
        deserialize(in,root->right);
    }