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;}
};