【LeetCode】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],
]

http://oj.leetcode.com/problems/binary-tree-level-order-traversal-ii/

/**
 * 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<Integer> temp =new ArrayList<Integer>();
        Stack<ArrayList<Integer>> stack = new Stack<ArrayList<Integer>>();
        ArrayList<ArrayList<Integer>> re = new ArrayList<ArrayList<Integer>>();
        Queue<TreeNode> tempqueue = new LinkedList<TreeNode>();
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        if(root!=null){
            queue.offer(root);

            while(root!=null){
                temp.add(root.val);
                if(root.left!=null){
                    tempqueue.offer(root.left);
                }
                if(root.right!=null){
                    tempqueue.offer(root.right);
                }
                
                queue.poll();
                if(queue.size()==0){
                    queue=tempqueue;
                    tempqueue= new LinkedList<TreeNode>();
                    stack.push(temp);
                    temp=new ArrayList<Integer>();
                    root=queue.peek();
                }else{
                    root = queue.peek();
                }
            }
            while(!stack.isEmpty()){
                re.add(stack.peek());
                stack.pop();
            }
            
            return re;
        }else{
            return re;
        }        
        
        
    }
}