using System;

using System.Text;

using System.Collections;

using System.Diagnostics;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        string s = "AABBCCDD";

            //output: {AAB,ABB,AABB,BBC,BCC,BBCC,CCD,CDD,CCDD}

            Dictionary<string,int>  filtertedSubstring = new Dictionary<string, int>();

            int startindex = 0;

            int uniqueCharLength = 2;

            int i;

            for (i= uniqueCharLength; i<s.Length; i++)

            {

                string sub = string.Empty;

                if (startindex + i <= s.Length)

                    sub = s.Substring(startindex, i);

                else continue;

                Hashtable hst = new Hashtable();

                foreach (char c in sub)

                {

                    if (hst.ContainsKey(c))

                        hst[c] = Convert.ToInt32(hst[c]) + 1;

                    else

                        hst.Add(c, 1);

                }

                //Fetch unique characters list.

                List<object> uniqueCharacters = new List<object>();

                foreach (var key in hst)

                {

                    uniqueCharacters.Add(key);

                }

                if (uniqueCharacters.Count == uniqueCharLength)

                    //Add the substring length to the list.

                    filtertedSubstring.Add(sub, sub.Length);

​

                else if (uniqueCharacters.Count > uniqueCharLength)

                {

                    startindex++;

                    i = uniqueCharLength;//reset the i value to traverse again from next index.

                }

            }

            //Get the hight count from the list;

            if(filtertedSubstring.Count>0)

                filtertedSubstring.OrderByDescending(f=> f.Value).FirstOrDefault();

          foreach(var lst in filtertedSubstring)

            {

                Console.WriteLine(lst);

            }

    }

}