using System;

using System.Collections.Generic;

using System.Linq;

​

namespace ConsoleApp

{

    public class Program

    {

        static void Main(string[] args)

        {

            var a = new[] { "a", "b" };

            var b = new[] { "1", "2", "3" };

            foreach (var strs in Interleave(a, b))

            {

                foreach (var str in strs)

                {

                    Console.Write(str);

                }

                Console.WriteLine();

            }

        }

​

        static IEnumerable<IEnumerable<T>> Interleave<T>(T[] a, T[] b)

        {

            if (a.Length == 0)

            {

                yield return b;

            }

            else if (b.Length == 0)

            {

                yield return a;

            }

            else

            {

                foreach (var rest in Interleave(a[1..], b))

                {

                    yield return Enumerable.Concat(new[] { a[0] }, rest);

                }

                foreach (var rest in Interleave(a, b[1..]))

                {

                    yield return Enumerable.Concat(new[] { b[0] }, rest);

                }

            }

        }

    }

}

​