怎么用TreeView分类3级比如：

TreeView1.Nodes.Clear();
        TreeNode tn = new TreeNode();
        tn.Text = "山东商务职业学院";
        TreeView1.Nodes.Add(tn);
        DataSet ds = GetDataset("select * from zz where 父字段='山东商务职业学院'");
        for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
        {
            TreeNode tn2 = new TreeNode();
            tn2.Text = ds.Tables[0].Rows[i]["字段"].ToString();

            //实际上只是把这一块重复了一遍,再下一级就再写一遍....层次深就不行了,理想的方法是写成递归
            DataSet sonds = GetDataset("select * from zz where 父字段='"+tn2.Text+"'");
             for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
            {
                TreeNode sontn2 = new TreeNode();
                sontn2.Text = sonds.Tables[0].Rows[i]["字段"].ToString();
                tn2 .ChildNodes.Add(sontn2);
             }
            tn.ChildNodes.Add(tn2);
        }

用递归