using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public class ReportingLineList

    {

        public int UserId { get; set; }

        public int? SupervisorUserId { get; set; }

    }

    public static void Main()

    {

        List<ReportingLineList> employeeList = new()

        {

            new() { UserId = 10 },

            new() { UserId = 1, SupervisorUserId = 10 },

            new() { UserId = 2, SupervisorUserId = 1 },

            new() { UserId = 3, SupervisorUserId = 1 },

            new() { UserId = 4, SupervisorUserId = 3 },

            new() { UserId = 5, SupervisorUserId = 10 },

            new() { UserId = 6, SupervisorUserId = 5 },

            new() { UserId = 7, SupervisorUserId = 6 },

            new() { UserId = 8, SupervisorUserId = 6 },

            new() { UserId = 9, SupervisorUserId = 8 }

        };

        var employeesWithReportingLevel = employeeList

            .Select(employee => new

                {

                    UserId = employee.UserId,

                    ReportingLevel = GetReportingLevel(employeeList, employee.UserId)

                })

            .OrderBy(reportingEmployee => reportingEmployee.ReportingLevel)

            .ToList();

        Console.WriteLine($"GetReportingLevel() was called {_getReportingLevelCallCount} times");

        Console.WriteLine();

        Console.WriteLine("UserId | ReportingLevel");

        Console.WriteLine("-------|----------------");

        foreach (var employee in employeesWithReportingLevel)

        {

            Console.WriteLine($"{employee.UserId.ToString().PadLeft(6, ' ').PadRight(6, ' ')} | {employee.ReportingLevel}");

        }

    }

    private static int _getReportingLevelCallCount;

    private static int GetReportingLevel(IEnumerable<ReportingLineList> employeeList, int userId)

    {

        _getReportingLevelCallCount++; // Not necessary for the calculation of the reporting level

        var subordinates = employeeList

            .Where(emp => emp.SupervisorUserId == userId);

        if (!subordinates.Any())

        {

            return 0;

        }

        return 1 + subordinates.Max(sub => GetReportingLevel(employeeList, sub.UserId));

    }

}

​