LeetCoder - 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],
]

/**
 * 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>> ret = new ArrayList<ArrayList<Integer>>();
        if(root==null) {
            return ret;
        }
        ArrayList<TreeNode> fronties = new ArrayList<TreeNode>();
        fronties.add(root);
        while(fronties.size()!=0) {
            ArrayList<Integer> level = new ArrayList<Integer>();
            ArrayList<TreeNode> tmp = new ArrayList<TreeNode>();
            for(TreeNode frontie : fronties) {
                level.add(frontie.val);
                if(frontie.left!=null) tmp.add(frontie.left);
                if(frontie.right!=null) tmp.add(frontie.right);
            }
            ret.add(0, level);
            fronties = tmp;
        }
        return ret;
    }
}