using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var list = new List<Course>();

​

        list.Add(new Course { Id = 1 });

        list.Add(new Course { Id = 2, RequiredCourse = "3" });

        list.Add(new Course { Id = 3, RequiredCourse = "4" });

        list.Add(new Course { Id = 4 });

        list.Add(new Course { Id = 5 });

        list.Add(new Course { Id = 6 });

        list.Add(new Course { Id = 7 });

        list.Add(new Course { Id = 8, RequiredCourse = "10" });

        list.Add(new Course { Id = 9 });

        list.Add(new Course { Id = 10 });

        list = list.OrderBy(i => i.Id).ThenBy(i => i.RequiredCourse).ToList();

​

        var copy = new List<Course>();

        foreach(var tt in list)

        {

            //Console.WriteLine(tt.Id + " " + tt.RequiredCourse);

            populateParent(list,ref copy, tt);

            if(!copy.Any(p => p.Id == tt.Id))

                copy.Add(tt);

        }

        foreach(var tt in copy)

        {

            Console.WriteLine(tt.Id + " " + tt.RequiredCourse);

        }

    }

    public static Course populateParent(List<Course> inputList,ref List<Course> outputList, Course current)

    {

        if(string.IsNullOrWhiteSpace(current.RequiredCourse))

        {

            if(!outputList.Any(p => p.Id == current.Id))

                outputList.Add(current);

            return current;

        }

        else

        {

            var tt = inputList.First(p => current.RequiredCourse != null && p.Id == int.Parse(current.RequiredCourse) );

            var parent = populateParent(inputList,ref outputList,tt);

            if(!outputList.Any(p => p.Id == parent.Id))

                outputList.Add(parent);

            if(!outputList.Any(p => p.Id == tt.Id))

                outputList.Add(tt);

            return tt;

        }   

    }   

}

​

public class Course

{

    public  int Id;

    public string RequiredCourse;

}