给定层序遍历求二叉树每层最大值的C++代码，横线处应填入哪个选项？
#include <vector>
#include <queue>
#include <algorithm>
struct TreeNode {
int val;
TreeNode* left;
TreeNode* right;
TreeNode(int x): val(x), left(nullptr), right(nullptr) {}
};
vector<int> largestValues(TreeNode* root) {
vector<int> result;
if (!root) return result;
queue<TreeNode*> q;
q.push(root);
while (!q.empty()) {
int sz = q.size();
int maxVal = INT_MIN;
for (int i = 0; i < sz; ++i) {
TreeNode* node;
_____
maxVal = max(maxVal, node->val);
if (node->left) q.push(node->left);
if (node->right) q.push(node->right);
}
result.push_back(maxVal);
}
return result;
}