using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        string s = "aaabcdd";

        int left = 0;

        int longest =  0;

        Dictionary<char, int> lookUp = new Dictionary<char, int>();

​

        for(int right = 0; right < s.Length; right++)

        {

            char c = s[right];

            if (lookUp.ContainsKey(c))      // character already found

            {

                left = lookUp[c];           // move left pointer to next to previously found location

                lookUp[c] = right + 1;      // update current location for the character

            }

            else

            {

                lookUp.Add(c, right + 1);   // character not found. so store index for this char. Note, storing next location

            }

            longest = Math.Max(longest, right - left + 1);

        }

        Console.WriteLine(longest);

    }

}