题目：

题解：

class Solution {
public:
    vector<vector<int>> levelOrder(Node* root) {
        if (!root) {
            return {};
        }

        vector<vector<int>> ans;
        queue<Node*> q;
        q.push(root);

        while (!q.empty()) {
            int cnt = q.size();
            vector<int> level;
            for (int i = 0; i < cnt; ++i) {
                Node* cur = q.front();
                q.pop();
                level.push_back(cur->val);
                for (Node* child: cur->children) {
                    q.push(child);
                }
            }
            ans.push_back(move(level));
        }

        return ans;
    }
};