void dfs(TreeNode* root) {
    if (root == nullptr)
        return;

    stack<TreeNode*> s;
    s.push(root);
    while (!s.empty()) {
        ________________________ // 在此处填入代码
        cout << node->value << " ";

        if (node->right) s.push(node->right);
        if (node->left) s.push(node->left);
    }
}