算法题：二叉树层序遍历，进一步提问：要求每层打印出一个换行符？

考察点：二叉树

publicList<List<Integer>> levelOrder(TreeNode root) {        List<List<Integer>> res =new ArrayList<List<Integer>>();        linkedList<TreeNode> queue = new linkedList<TreeNode>();        if (root == null) { return res;        }        queue.offer(root);        while (queue.size() != 0) { List<Integer> l = new ArrayList<Integer>(); int size = queue.size(); for (int i = 0; i < size; i++) {     TreeNode temp = queue.poll();     l.add(temp.val);     if (temp.left != null) {         queue.offer(temp.left);     }     if (temp.right != null) {         queue.offer(temp.right);     } } res.add(l);        }        return res;}