[LeetCode] Binary Tree Level Order traversal II

Question:

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],
]

Java Code:

public List> levelOrderBottom(TreeNode root) {
        List> res = new ArrayList>();
        Queue tmp = new LinkedList();
        Stack> res0 = new Stack>();
        
        if(root == null) return res;
        
        tmp.add(root);
        
        while(!tmp.isEmpty()){
            int size = tmp.size();
            List level = new ArrayList();
            
            for(int i = 0; i < size; i++){
                TreeNode node = tmp.poll();
                level.add(node.val);
                
                if(node.left != null) tmp.add(node.left);
                if(node.right != null) tmp.add(node.right);
            }
            
            res0.add(level);
        }
        
        while(!res0.isEmpty()){
            res.add(res0.pop());
        }
        
        return res;
    }