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

<pre><code class=""language-java"" lang=""java"">public
List<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;
}
</code></pre>

<pre><code> "“`