binary tree level order traversal

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> lists = new ArrayList<List<Integer>>();
        if (root == null) return lists;
        Deque<TreeNode> queue1 = new ArrayDeque<TreeNode>();
        Deque<TreeNode> queue2 = new ArrayDeque<TreeNode>();
        queue1.offer(root);
        while (!queue1.isEmpty() || !queue2.isEmpty()){
            List<Integer> list = new ArrayList<Integer>();
            List<Integer> list2 = new ArrayList<Integer>();

            while (!queue1.isEmpty()){
                TreeNode node = queue1.poll();
                list.add(node.val);
                if (node.left != null){
                    queue2.offer(node.left);
                }
                if (node.right != null){
                    queue2.offer(node.right);
                }
            }

            if (!list.isEmpty()){
                lists.add(list);
            }

            while (!queue2.isEmpty()){
                TreeNode node = queue2.poll();
                list2.add(node.val);
                if (node.left != null){
                    queue1.offer(node.left);
                }
                if (node.right != null){
                    queue1.offer(node.right);
                }
            }

            if (!list2.isEmpty()){
                lists.add(list2);
            }
        }
     return lists;
    }
}