Average value by level in a tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res=new LinkedList<>();
        if(root==null) return res;
        Queue<TreeNode> queue=new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()){
            int size=queue.size();
            List<Integer> list=new LinkedList<>();
            for(int i=0;i<size;i++){
                TreeNode cur=queue.poll();
                list.add(cur.val);
                if(cur.left!=null) queue.offer(cur.left);
                if(cur.right!=null) queue.offer(cur.right);
            }
            res.add(list);
        }
        return res;
    }
}