using System;

​

namespace ArrayMerge

{

    class Program

    {

        static void Main(string[] args)

        {

            var a = new int[] { 1, 2, 5, 8, 10 };

            var b = new int[] { 1, 3, 5, 7, 13 };

            var res = ArrayMerge(a,b);

            foreach(var i in res)

            {

                Console.Write($"{i} ");

            }

​

        }

​

        static int[] ArrayMerge(int[] a, int[] b)

        {

            var res = new int[a.Length + b.Length];

            return ArrayMergeRecursion(a,b,0,0,res);

        }

        // To iterate is human, to recurse, divine.

        static int[] ArrayMergeRecursion(int[] a, int[] b, int i, int j, int[] c)

        {

            if (i < a.Length || j < b.Length)

            {

                if (i >= a.Length)

                {

                    for (int k = j; k < b.Length; k++)

                    {

                        c[i+k] = b[k];

                    }

                    return c;

                }

                else if (j >= b.Length)

                {

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

                    {

                        c[j+k] = a[k];

                    }

                    return c;                    

                }

                else

                {

                    if (a[i] < b[j])

                    {

                        c[i+j] = a[i];

                        return ArrayMergeRecursion(a,b,i+1,j,c);

                    }

                    else

                    {

                        c[i+j] = b[j];

                        return ArrayMergeRecursion(a,b,i,j+1,c);                        

                    }

                }

            }

            else

            {

                return c;

            }

        }

    }

}

​