Leetcode Binary Tree Level Order Traversal II

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if(root == null){
            return result;
        }
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        Stack<List<Integer>> s = new Stack<List<Integer>>();
        
        queue.add(root);
        while(!queue.isEmpty()){
            int size = queue.size();
            List<Integer> numberList = new ArrayList<Integer>();
            while(size > 0){
                TreeNode x = queue.remove();
                numberList.add(x.val);
                if(x.left != null) {
                    queue.add(x.left);
                }
                if(x.right != null){
                    queue.add(x.right);
                }
                size--;
            }
            s.push(numberList);
        }
         while(!s.isEmpty()){
             result.add(s.pop());
         }
         return result;
    }
}