Binary Tree BFS Template

public class Solution {    public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {        ArrayList result = new ArrayList();                if (root == null)            return result;                    Queue<TreeNode> queue = new LinkedList<TreeNode>();        queue.offer(root);                while (!queue.isEmpty()) {            ArrayList<Integer> level = new ArrayList<Integer>();            int size = queue.size();            for (int i = 0; i < size; i++) {                TreeNode head = queue.poll();                level.add(head.val);                if (head.left != null)                    queue.offer(head.left);                if (head.right != null)                    queue.offer(head.right);            }            result.add(level);        }                return result;    }}

需要一个辅助队列