Binary Tree Level Order Traversal II

Given a binary tree, return the bottom-up level order traversal of its nodes' values.
(ie, from left to right, level by level from leaf to root).
For example:
Given binary tree [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its bottom-up level order traversal as:
[ [15,7], [9,20], [3] ]

题目大意：返回树的层次遍历，每一层从左到右，从叶子层到根

题目难度: Medium

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

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

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

            List<Integer> item = new ArrayList<>();
            while (size-- > 0) {
                TreeNode top = queue.poll();
                item.add(top.val);
                if (top.left != null) queue.offer(top.left);
                if (top.right != null) queue.offer(top.right);
            }
            result.add(0,item);
        }
    }
}