题目：

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代码(首刷自解 2024年1月24日）：

/*
// Definition for a Node.
class Node {
public:
    int val;
    vector<Node*> children;

    Node() {}

    Node(int _val) {
        val = _val;
    }

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/

class Solution {
public:
    vector<vector<int>> levelOrder(Node* root) {
        vector<vector<int>> res = {};
        if(root == nullptr) return res;
        queue<Node*> que;
        Node* cur = root;
        que.push(cur);
        int size = 0;
        while (!que.empty()) {
            size = que.size();
            vector<int> vec;
            while (size--) {
                Node* node = que.front();
                que.pop();
                vec.emplace_back(node->val);
                for(int i = 0; i < node->children.size(); ++i){
                    que.push(node->children[i]);
                }
            }
            res.emplace_back(vec);
        }
        return res;
    }
};