class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> ans=new ArrayList<>();
        if(root==null) return ans;
        
        List<Integer> level=new ArrayList<>();
        
        Queue<TreeNode> q=new LinkedList<>();
        q.offer(root);
        q.offer(null);
        
        while(!q.isEmpty()){
            TreeNode temp=q.poll();
            if(temp==null){
                ans.add(level);
                level.clear();
                if(!q.isEmpty()){
                    q.offer(null);
                }
            } else {
                level.add(temp.val);
                if(temp.left!=null) q.offer(temp.left);
                if(temp.right!=null) q.offer(temp.right);
            }
        } 
        return ans;
    }
}