CustomMerge, C# - rextester

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            string[] t1 = new string[] { "a", "b", "c" };
            string[] t2 = new string[] { "a1", "a2", "b1", "b2", "c1", "c2" };
            List<string> merged = Merge(t1.ToList(), t2.ToList());
            foreach (string item in merged)
            {
                Console.WriteLine(item);
            }
        }

private static List<T> Merge<T>(List<T> first, List<T> second)
        {
            List<T> ret = new List<T>();
            for (int indexFirst = 0, indexSecond = 0; 
                indexFirst < first.Count && indexSecond < second.Count; 
                indexFirst++, indexSecond+= 2)
            {
                ret.Add(first[indexFirst]);
                ret.Add(second[indexSecond]);
                ret.Add(second[indexSecond + 1]);
            }
            return ret;
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,2 sec, absolute running time: 0,13 sec, cpu time: 0,11 sec, average memory usage: 14 Mb, average nr of threads: 3, absolute service time: 0,36 sec