using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        var flights = new List<Flight>()

        {

            new Flight{OperatingAirline="TX",IsUnitedExpress=true},

            new Flight{OperatingAirline="LH",IsUnitedExpress=false},

            new Flight{OperatingAirline="GG",IsUnitedExpress=true},

            new Flight{OperatingAirline="UA",IsUnitedExpress=false},

            new Flight{OperatingAirline="LH",IsUnitedExpress=false},

            new Flight{OperatingAirline="SW",IsUnitedExpress=true}

        };

​

        var carrierCodes = flights.Select(x => x.OperatingAirline).Distinct().ToList();

​

        if (flights.Any(x => x.IsUnitedExpress))

            carrierCodes.Add("UAX");

        if (flights.Where(x => x.OperatingAirline == "UA").All(x => x.IsUnitedExpress))

            carrierCodes.Remove("UA");

​

        var result = string.Join("", carrierCodes.OrderBy(x => x));

​

        Console.WriteLine($"result is {result}");

    }

}

​

public class Flight

    {

        public string DepartureDateTime { get; set; }

        public string ArrivalDateTime { get; set; }

        public string OperatingAirline { get; set; }

        public bool IsUnitedExpress { get; set; }

    }