using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

​

​

public class Program

{

    public static void Main()

    {

        List<string> owels = new List<string>(){"a","e","i","o","u"};

        var inputs = new List<string>(){"A","x","nI","dO","Uh","Eo","gI","gO","Ui","Ui","Ui","Ui"};

        //var startswith = inputs.Join(owels, input => input, ovel => ovel, (input,ovel) = > new {i = i, o=o}).Where()

        var startswith = (from i in inputs

            join o in owels

            on i.Substring(0,1).ToLower() equals o

            select i).ToList();

        var notstarts = inputs.Except(startswith).ToList();

        Console.WriteLine(String.Join(",", startswith));

        Console.WriteLine(String.Join(",", notstarts));

        List<string> output = new List<string>();

        int pos = 0;

        while(startswith.Any() || notstarts.Any())

        {

            if(pos%2==0)

            {

                if(startswith.Any())

                {

                    var temp = startswith.First();

                    startswith.Remove(temp);

                    output.Add(temp);

                }

            }else if (notstarts.Any()){

                    var temp = notstarts.First();

                    notstarts.Remove(temp);

                    output.Add(temp);

            }else{output.Add(null);}

            pos++;

        }

        Console.WriteLine(String.Join(",", output));

    }

}