using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        int[] a = { 1, 2, 3, 4 };

        int[] b = {6,5,4,3,2};

        int[] c = PerfectShuffle(a, b);

        c.Dump();

    }

    public static int[] PerfectShuffle(int[] a, int[] b)

    {

        if( a == null || !a.Any()) return b ?? new int[0];

        if( b == null || !b.Any()) return a ?? new int[0];

        List<int> c = new List<int>();

        for(int i = 0; i < a.Length; i++)

        {

            c.Add(a[i]);

            if(i < b.Length)

                c.Add(b[i]);

        }

        if(b.Length > a.Length)

            c.AddRange(b.Skip(a.Length).ToArray());

        return c.ToArray();

    }

}