C#-Code

private string ReorderString(string input)
        {
            // Anteile Buchstaben und Ziffern
            var alphas = Regex.Matches(input, @"[a-z]");
            var nums = Regex.Matches(input, @"[0-9]");

            // Prüfen, ob gleich beide lang oder maximal 1 Unterschied, sonst leeren String zurück geben
            if (Math.Abs(alphas.Count - nums.Count) > 1)
                return "";

            // Herausfinden, ob mehr Zahlen oder Buchstaben enthalten sind
            var biggerCollection = alphas;
            var smallerCollection = nums;

            if (alphas.Count < nums.Count)
            {
                biggerCollection = nums;
                smallerCollection = alphas;
            }

            // Ergebnis-String zusammensetzen (gleiche Länge)
            string result = "";

            for (int i = 0; i < smallerCollection.Count; i++)
            {
                result += biggerCollection[i];
                result += smallerCollection[i];
            }

            // Falls ungleiche Länge, dann das letzte fehlende Zeichen hinzufügen
            if (biggerCollection.Count != smallerCollection.Count)
                result += biggerCollection[biggerCollection.Count - 1];

            return result;
        }

C#-Code

using System;
using System.Collections.Generic;
using System.Linq;

var lst = new List<string>()
{
    "train1234", "1234567", "abcdefg", "abcdef123", "abcd1234"
};

lst.ForEach(x => Console.WriteLine(OrderedSequence(x)));

string OrderedSequence(string s)
{
    char[] arr = new char[s.Length];

    int nc = s.Count(x => x >= 'a' && x <= 'z');
    int nd = s.Count(x => x >= '0' && x <= '9');
    int cc = 0;
    int cd = 0;

    if (Math.Abs(nc - nd) > 1) return "";
    if (nc > nd)
        cd = 1;
    else
        cc = 1;

    foreach (char i in s)
    {
        if (i >= 'a' && i <= 'z')
        {
            arr[cc] = i;
            cc += 2;
        }
        else
        {
            arr[cd] = i;
            cd += 2;
        }
    }

    return new string(arr);
}

C#-Code

using System;
using System.Linq;

namespace TestFramwork
{
    public class Program
    {
        public static void Main()
        {
            try
            {
                string[] test = new string[] { "train1234", "1234567", "abcdefg", "abcdef123", "abcd1234", "abcd12345" };
                foreach(string s in test)
                {
                    Console.WriteLine($"Ergebniss zu:\ninput: {s}\noutput: {HandleString(s)}\n\n");
                }

            }
            catch (Exception ex)
            {
                string error = ex.ToString();
            }
        }

        private static string HandleString(string input)
        {

            int countLetter = input.ToCharArray().Count(c => char.IsLetter(c));
            int countDigit = input.ToCharArray().Count(c => char.IsDigit(c));

            if ((countLetter + 1 == countDigit || countLetter - 1 == countDigit) || countDigit == countLetter)
            {
                string result = "";

                //Deppends on which Counter is longer we set the chars
                int charIndex1 = countLetter < countDigit ? countLetter : 0;
                int charIndex2 = countLetter - charIndex1;
                bool charIndex1IsLetter = charIndex1 == 0;

                //FOR -> As long as the longest Counter is (Digit, Letter)
                for (int i = 0; i < (countLetter < countDigit ? countDigit : countLetter); i++)
                {
                    //To avoid OutOfIndexEx check the possibility
                    if (charIndex1 == input.Length 
                        || !(charIndex1IsLetter ? char.IsLetter(input[charIndex1]) : char.IsDigit(input[charIndex1])))
                    {
                        result += input[charIndex2].ToString();
                    }
                    else if (charIndex2 == input.Length 
                        || !(charIndex1IsLetter ? char.IsDigit(input[charIndex2]) : char.IsLetter(input[charIndex2])))
                    {
                        result += input[charIndex1].ToString();
                    }
                    else if (char.IsLetter(input[charIndex1IsLetter ? charIndex1 : charIndex2])
                        && char.IsDigit(input[!charIndex1IsLetter ? charIndex1 : charIndex2]))
                    {
                        result += input[charIndex1].ToString() + input[charIndex2].ToString();
                    }
                    else
                    {
                        return "";
                    }
                    charIndex1++;
                    charIndex2++;
                }
                return result;
            }

            return "";
        }
    }



}

C#-Code

Console.Write("Bitte schreibe ein Wort der gleichen Anzahl von Buchstaben und Ziffer hat: ");
            string s = Console.ReadLine();
            Abwechseln(s);
        }

        static void Abwechseln(string Text)
        {
            int buchstaben = 0;
            int ziffer = 0;
            bool buchstabenGleichZiffer = false;
            string antwort = "";
            string buchstabenString = "";
            string zifferString = "";
            // Prüfen ob Buchstaben und Zahlen gleich sind.

            for (int i = 0; i < Text.Length; i++)
            {
                if (Char.IsDigit(Text[i]))
                {
                    ziffer++;
                }
                else
                {
                    buchstaben++;
                }
            }

            if (buchstaben == ziffer) // prüfe ob buchstabenAnzahl gleich zifferAnzahl ist.
            {
                buchstabenGleichZiffer = true;
                buchstabenString = Text.Substring(0, buchstaben);
                zifferString = Text.Substring(ziffer);
            }
            

            if (buchstabenGleichZiffer)
            {
                for (int i = 0; i < buchstabenString.Length; i++)
                {
                    antwort = antwort + buchstabenString[i] + zifferString[i];
                }
            }
            Console.WriteLine();
            Console.WriteLine(antwort);
            Console.ReadLine();