下列代码实现了树的深度优先遍历，则横线处应填入（ ）。

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x): val(x), left(nullptr), right(nullptr) {}
};

void dfs(TreeNode* root) {
    if (!root) return;
    stack<TreeNode*> st;
    st.push(root);
    while (!st.empty()) {
        TreeNode* node = st.top(); st.pop();
        cout << node->val << " ";
        if (node->right) st.push(node->right);
        ________________________
    }
}

{{ select(1) }}

• if (node->left) st.push(node->left);
• if (node->left) st.pop(node->left);
• if (node->left) st.front(node->left);
• if (node->left) st.push(node->right);