class Solution {
    public int[] levelOrder(TreeNode root) {
        if(root == null) return new int[0];
        Deque<TreeNode> queue = new ArrayDeque<>();
        queue.offer(root);
        // Deque<TreeNode> queue = new ArrayDeque<>() {{offer(root);}};  // 匿名内部类初始化
        List<Integer> res = new ArrayList<>();

        while(!queue.isEmpty()){
            TreeNode cur = queue.poll();
            res.add(cur.val);
            if(cur.left != null) queue.offer(cur.left);
            if(cur.right != null) queue.offer(cur.right);
        }

        return res.stream().mapToInt(x -> x).toArray();
    }
}