public List
     
       preorderTraversal(TreeNode root) {        List
      
        results = new ArrayList
       
        ();        if(root == null)            return results;                Deque
        
          unprocessed = new ArrayDeque
         
          ();        TreeNode current = root;                while(current != null || !unprocessed.isEmpty())        {            if(current == null)                current = unprocessed.pop();                            results.add(current.val);            if(current.right!=null)                unprocessed.push(current.right);            current = current.left;                    }        return results;    }