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,#,#,15,7},

    3   / \  9  20    /  \   15   7

return its bottom-up level order traversal as:

[  [15,7]  [9,20],  [3],]

Analysis: BFS. 

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();        Queue<TreeNode> frontier = new LinkedList<TreeNode>();                if(root==null) return res;        frontier.add(root);        while(!frontier.isEmpty()) {            Queue<TreeNode> next = new LinkedList<TreeNode>();            ArrayList<Integer> cell = new ArrayList<Integer>();            for(int i=frontier.size()-1; i>=0; i--) {                TreeNode current = frontier.poll();                if(current.left!=null) next.add(current.left);                if(current.right!=null) next.add(current.right);                cell.add(current.val);            }            res.add(0, cell);            frontier = next;        }        return res;    }}