Боря Фролов donRumatta

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

namespace ConsoleApp2
{
    static class Program
    {
        static void Main(string[] args)
        {
            CountSequences("aabccaaab");
            CountSequences("abbaaaa");
            CountSequences("aabbbccccaaaccccccca");
            CountSequences("");
            CountSequences("abababaccc");
            CountSequences("cabccaabbcccaaabbb");
            CountSequences("cabccaabbcccaaabbbcab");

            Console.ReadKey();
        }

        private static void CountSequences(string input)
        {
            var result = input
                .Distinct()
                .Select(x => 
                    new KeyValuePair<char, int>(
                        x, 
                        input.ReverseSplit(x).Max(y => y.Length)));

            var formatted = result.Select(x => $"{x.Key} - {x.Value}");
            Console.WriteLine($"{input}: {string.Join(", ", formatted)}");
        }

        public static List<string> ReverseSplit(this string input, char nonSeparator)
        {
            var result = new List<string>();

            var sb = new StringBuilder();

            void FinalizeSequence()
            {
                if (sb.Length > 0)
                {
                    result.Add(sb.ToString());

                    sb.Clear();
                }
            };

            var i = 0;
            while (i < input.Length)
            {
                if (input[i] == nonSeparator)
                {
                    sb.Append(nonSeparator);
                }
                else
                {
                    FinalizeSequence();
                }

                i++;
            }

            FinalizeSequence();

            return result;
        }
    }
}