429.N叉树的层次遍历

class Solution {
    public List<List<Integer>> levelOrder(Node root) {
        List<List<Integer>> res = new LinkedList<>();
        if(root == null) return res;
        Queue<Node> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()) {
            List<Integer> list = new LinkedList<>();
            int sz = queue.size();
            for(int i = 0; i < sz; i++) {
                list.add(queue.peek().val); //返回首部
                queue.addAll(queue.poll().children); //移除首部后的子树
            }
            res.add(list);
        }
        return res;
    }
}