longest non-repeating substring

namespace LongestSubstring
{
    using System;

    internal class Program
    {
        // This function returns true if all characters in
        // str[i..j] are distinct, otherwise returns false
        public static bool AreDistinct(string str, int i, int j)
        {

            // Note : Default values in visited are false
            bool[] visited = new bool[26];

            for (int k = i; k <= j; k++)
            {
                if (visited[str[k] - 'a'] == true)
                {
                    return false;
                }

                visited[str[k] - 'a'] = true;
            }
            return true;
        }

        // Returns length of the longest substring
        // with all distinct characters.
        public static int LongestUniqueSubsttr(string str)
        {
            int n = str.Length;

            // Result
            int res = 0;

            for (int i = 0; i < n; i++)
            {
                for (int j = i; j < n; j++)
                {
                    if (AreDistinct(str, i, j))
                    {
                        res = Math.Max(res, j - i + 1);
                    }
                    else
                    {
                        // note: newly added to avoid going n time when there is a duplication in some n[i]
                        i = j - 1; // it will be adjusted by the for loop
                        break;
                    }
                }
            }

            return res;
        }

        // Driver code
        public static void Main()
        {
            string str = "geeksforgeeks";
            Console.WriteLine("The input string is " + str);

            int len = LongestUniqueSubsttr(str);

            Console.WriteLine("The length of the longest " +
                              "non-repeating character " +
                              "substring is " + len);

            Console.ReadLine();
        }
    }
}