using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Building lists:");

        PrintList(Interleave("ABC".ToCharArray().ToList(), "123".ToCharArray().ToList()));

        PrintList(Interleave("ABCDE".ToCharArray().ToList(), "123".ToCharArray().ToList()));

        PrintList(Interleave("ABC".ToCharArray().ToList(), "12345".ToCharArray().ToList()));

        Console.WriteLine();

        Console.WriteLine("Walking through enumerators:");

        PrintList("ABC".ToCharArray().InterleaveWith("123".ToCharArray()));

        PrintList("ABCDE".ToCharArray().InterleaveWith("123".ToCharArray()));

        PrintList("ABC".ToCharArray().InterleaveWith("12345".ToCharArray()));

    }

    public static void PrintList<T>(IEnumerable<T> items)

    {

        Console.WriteLine(string.Join(",", items));

    }

    public static List<T> Interleave<T>(List<T> male, List<T> female)

    {

        var children = new List<T>();

        var childrenCount = male.Count + female.Count;

        var indexMale = 0;

        var indexFemale = 0;

        for (int i = 0; i < childrenCount; i++)

        {

            if (i % 2 == 1)

            {

                if (indexMale < male.Count)

                {

                    children.Add(male[indexMale]);

                    indexMale++;

                }

                else

                {

                    children.Add(female[indexFemale]);

                    indexFemale++;

                }

            }

            else

            {

                if (indexFemale < female.Count)

                {

                    children.Add(female[indexFemale]);

                    indexFemale++;

                }

                else

                {

                    children.Add(male[indexMale]);

                    indexMale++;

                }

            }

        }

        return children;

    }

}

​

public static class IEnumerableExtensions

{

    public static IEnumerable<T> InterleaveWith<T>(this IEnumerable<T> sequenceA, IEnumerable<T> sequenceB)

    {

        var enumA = sequenceA.GetEnumerator();

        var enumB = sequenceB.GetEnumerator();

        // As long as there are elements in sequence A

        while (enumA.MoveNext())

        {

            // Take an element from sequence A

            yield return enumA.Current;

            // And, if possible,

            if (enumB.MoveNext())

            {

                // Take an element from sequence B

                yield return enumB.Current;

            }

        }

        // If there are any elements left over in sequence B

        while (enumB.MoveNext())

        {

            // Take each of them

            yield return enumB.Current;

        }

    }

}