给定一个二叉树，返回每一层中最大的节点值，结果以数组形式返回，横线处应填入（ ）。

1#include <vector>2#include <queue>3#include <algorithm>4 5struct TreeNode {6   int val;7   TreeNode* left;8   TreeNode* right;9   TreeNode(int x): val(x), left(nullptr), right(nullptr) {}10};11 12vector<int> largestValues(TreeNode* root) {13   vector<int> result;14   if (!root) return result;15 16   queue<TreeNode*> q;17   q.push(root);18 19   while (!q.empty()) {20     int sz = q.size();21     int maxVal = INT_MIN;22     for (int i = 0; i < sz; ++i) {23        TreeNode* node;24        _______________________________25        maxVal = max(maxVal, node->val);26        if (node->left) q.push(node->left);27        if (node->right) q.push(node->right);28     }29     result.push_back(maxVal);30   }31 32   return result;33}