Find longest substring without repeating characters

Question

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

Example:

Input:

Answer 1

I modified my solution to "find the length of the longest substring without repeating characters".

        public string LengthOfLongestSubstring(string s) {
    var res = 0;
    var dict = new Dictionary();
    var start = 0;

    for(int i =0; i< s.Length; i++)
    {
        if(dict.ContainsKey(s[i]))
        {
            start = Math.Max(start, dict[s[i]] + 1); //update start index
            dict[s[i]] = i;
        }
        else
        {
            dict.Add(s[i], i);
        }

        res = Math.Max(res, i - start + 1);  //track max length
    }
    return s.Substring(start,res);
}