public List<Integer> postorderTraversal(TreeNode root) {
        if(root == null) return postOrder;
        if(root.left != null)
        {
          postorderTraversal(root.left);
        }
        if(root.right != null)
        {
            postorderTraversal(root.right);
        }
        postOrder.add(root.val);
//        System.out.println(postOrder);
        return postOrder;
    }