Binary Tree Level Order Traversal II

public class Solution {
    public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(root == null)
            return result;
        Stack<ArrayList<Integer>> stack = new Stack<ArrayList<Integer>>();
        stack = traversal(root);
        while(!stack.isEmpty())
            result.add(stack.pop());
        return result;
    }
    private Stack<ArrayList<Integer>> traversal(TreeNode root)
    {
        LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
        Stack<ArrayList<Integer>> result = new Stack<ArrayList<Integer>>();
        queue.add(root);
        while(!queue.isEmpty())
        {
            ArrayList<Integer> tmp = new ArrayList<Integer>();
            LinkedList<TreeNode> tmpqueue = new LinkedList<TreeNode>();
            while(!queue.isEmpty())
            {
                TreeNode node = queue.poll();
                tmp.add(node.val);
                if(node.left != null)
                    tmpqueue.add(node.left);
                if(node.right != null)
                    tmpqueue.add(node.right);
            }
            result.add(tmp);
            queue = tmpqueue;
        }
        return result;
    }
}