如何把list中父子类对象递归成树

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication4
{
    public class TestModel
    {
        public void test()
        {
            #region 添加测试数据
            List<People> listP = new List<People>();
            for (int i = 0; i < 1; i++)
            {
                listP.Add(new People()
                {
                    ParentId = 0,
                    id = 1
                });
            }
            for (int i = 0; i < 2; i++)
            {
                listP.Add(new People()
                {
                    ParentId = 1,
                    id = i + 10,
                });
            }
            for (int i = 0; i < 3; i++)
            {
                listP.Add(new People()
                {
                    ParentId = 10,
                    id = i + 100, 
                });
            }
            for (int i = 0; i < 3; i++)
            {
                listP.Add(new People()
                {
                    ParentId = 11,
                    id = i + 100,
                });
            }
            #endregion 上面是添加测试数据
            
            //查询当前节点下的所有子孙对象
            var currentP = new People();
            currentP.id = 1;
           
            Recursion(listP, currentP);
            
        }

        public void Recursion(List<People> list,People p)
        {
            List<People> peoples = new List<People>();
            for (int i = 0; i < list.Count; i++)
            {
                //递归必须要有跳出节点，此处为了不断向下查找子节点
                if (list[i].ParentId == p.id)
                {
                    peoples.Add(list[i]);
                    p.PeopleList = peoples;
                    Recursion(list, list[i]);
                }
            }      
        }
    }
    public class People
    {
        public List<People> PeopleList;
        public int ParentId;
        public int id;
    }
}