The longest substring without repeating letters

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

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            var input = "abcdeabcdebb";
            var output = lengthOfLongestSubstring(input);
            Console.WriteLine(output);
            input = "bbbbaaa";
            output = lengthOfLongestSubstring(input);
            Console.WriteLine(output);
            Console.ReadLine();
            // The longest substring without repeating letters for "abcabcbb" is "abc", which the length is 3.
            // for "bbbbb" the longest substring "b" with the length of 1.
        }


        static int lengthOfLongestSubstring(string s)
        {
            int i = 0, j = 0;
            int maxLen = 0;

            // Set all characters as not-existing
            var exist = new bool[256];

            while (j < s.Length) // Get the length of string
            {
                // Check if the character exists
                if (exist[s[j]])
                {
                    maxLen = Math.Max(maxLen, j - i);
                    while (s[i] != s[j])
                    {
                        exist[s[i]] = false;
                        i++;
                    }

                    i++;
                    j++;
                }
                else
                {
                    exist[s[j]] = true;
                    j++;
                }
            }
            return maxLen = Math.Max(maxLen, s.Length - i);
        }
    }
}