题目

代码

if(root == null){
return new ArrayList<>();
}

    Queue<TreeNode> queue = new LinkedList<>();List<List<Integer>> res = new ArrayList<>();queue.add(root);while(!queue.isEmpty()){int k = queue.size();List<Integer> tmp = new ArrayList<>();for(int i = 0; i < k; i++){TreeNode t = queue.poll();tmp.add(t.val);if(t.left != null) queue.add(t.left);if(t.right != null) queue.add(t.right);}res.add(tmp);}return res;