Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes' values.
(ie, from left to right, then right to left for the next level and alternate between).
For example:
Given binary tree [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its zigzag level order traversal as:
[[3], [20,9], [15,7]]

题目大意：层次遍历，第一层从左到右，第二层从右到左，依次交换

题目难度：Medium

import java.util.*;
/**
 * Created by gzdaijie on 16/6/7
 */
public class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();

        if (root == null) return result;
        queue.add(root);

        boolean tag = true;
        while (true) {
            int size = queue.size();
            if (size == 0) return result;

            List<Integer> item = new LinkedList<>();
            while (size-- > 0) {
                TreeNode top = queue.poll();

                if (tag) item.add(top.val);
                else item.add(0, top.val);

                if (top.left != null) queue.offer(top.left);
                if (top.right != null) queue.offer(top.right);
            }
            result.add(item);
            tag = !tag;
        }
    }
}