using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        //Console.WriteLine("Hello World");

        var shortestExample = Shortest("abbaaaaaaabbbbbabc");

        Console.WriteLine(string.Join("\n\r",shortestExample));

    }

    private static string[] Shortest(string testString)

    {

     var k = SpecialSubstrings(testString);

     var min = k.Min(x => x.Count);

     return k.Where(x => x.Count == min).First().ToArray();

    }

    private static List<List<string>> SpecialSubstrings(string subString)

    {

        var subStrings = new List<List<string>>();

        if (string.IsNullOrEmpty(subString))

        {

            subStrings.Add(new List<string>());

            return subStrings;

        }

        var firstChar = subString[0];

        var pos = 0;

​

        while (pos >= 0)

        {

            var startOfString = subString.Substring(0, pos+1);

            foreach (var list in SpecialSubstrings(subString.Substring(pos + 1)))

            {

                list.Insert(0, startOfString);

                subStrings.Add(list);

            }

            pos = subString.IndexOf(firstChar, pos + 1);

        }

        return subStrings;

    }

}

​

​