java树形结构如何用递归实现？

public class Ttt {
	public String tree(List<Map> ret, String rootId, String p) {
		StringBuffer retList = new StringBuffer();
		for (Map m : ret) {
			if (rootId.equals(m.get("PARENTID"))) {
				retList.append(p+m.get("NAME")+"\n");
				retList.append(tree(ret, (String) m.get("ID"), p + "-"));
			}
		}
		return retList.toString();
	}

	public static void main(String[] args) {
		List<Map> list = new ArrayList<>();
		Map m = new HashMap<>();
		m.put("ID", "1");
		m.put("NAME", "a");
		m.put("PARENTID", "root");
		list.add(m);
		m = new HashMap<>();
		m.put("ID", "2");
		m.put("NAME", "b");
		m.put("PARENTID", "1");
		list.add(m);
		m = new HashMap<>();
		m.put("ID", "3");
		m.put("NAME", "c");
		m.put("PARENTID", "2");
		list.add(m);
		m = new HashMap<>();
		m.put("ID", "4");
		m.put("NAME", "d");
		m.put("PARENTID", "1");
		list.add(m);
		m = new HashMap<>();
		m.put("ID", "5");
		m.put("NAME", "e");
		m.put("PARENTID", "root");
		list.add(m);
		m = new HashMap<>();
		m.put("ID", "6");
		m.put("NAME", "f");
		m.put("PARENTID", "5");
		list.add(m);
		m = new HashMap<>();
		m.put("ID", "7");
		m.put("NAME", "g");
		m.put("PARENTID", "2");
		list.add(m);
		m = new HashMap<>();
		m.put("ID", "8");
		m.put("NAME", "h");
		m.put("PARENTID", "2");
		list.add(m);
		m = new HashMap<>();
		m.put("ID", "9");
		m.put("NAME", "i");
		m.put("PARENTID", "3");
		list.add(m);
		Ttt t = new Ttt();
		System.out.println(t.tree(list, "root", ""));
	}
}

运行效果

ID，parentid，数据库用start-with递归实现