Find longest substring without repeating characters

Question

Given a string S of length N find longest substring without repeating characters.

Example:

Input:

Answer 1

private static string LongestSubstring(string word)
        {
            var set = new HashSet();
            string longestOverAll = "";
            string longestTillNow = "";

            foreach (char c in word)
            {
                if (!set.Contains(c))
                {
                    longestTillNow += c;
                    set.Add(c);
                }
                else
                {
                    longestTillNow = string.Empty;
                }

                if (longestTillNow.Length > longestOverAll.Length)
                {
                    longestOverAll = longestTillNow;
                }
            }

            return longestOverAll;
        }