using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        GradeSchool gradeSchool = new GradeSchool();

        gradeSchool.Add("Peter", 2);

        gradeSchool.Add("Anna", 1);

        gradeSchool.Add("Barb", 1);

        gradeSchool.Add("Zoe", 2);

        gradeSchool.Add("Alex", 2);

        gradeSchool.Add("Jim", 3);

        gradeSchool.Add("Charlie", 1);

        foreach(var v in gradeSchool.Roster())

        {

            Console.WriteLine(v); // "Aimee", "Blair", "James", "Paul"

        }

    }

}

​

public class GradeSchool

{

    private SortedList<string, int> _roster;

    public GradeSchool()

    {

        _roster = new SortedList<string, int>();

    }

    public void Add(string student, int grade)

    {

        _roster.Add(student, grade);

    }

​

    public IEnumerable<string> Roster()

    {

        return _roster.OrderBy(x => x.Value).Select(x => x.Key);

    }

​

    public IEnumerable<string> Grade(int grade)

    {

        return _roster.Where(x => x.Value == grade).Select(x => x.Key);

    }

}