3 无重复最长字符串

public class Solution {
    public int LengthOfLongestSubstring(string s) {
          if (s == null || s == "")
                return 0;

            int n = s.Length;
            HashSet<char> hs = new HashSet<char>();
            int ans = 0, start = 0, stop = 0;
            while (start < n && stop < n)
            {
                if (!hs.Contains(s[stop]))
                {
                    hs.Add(s[stop++]);
                    ans = Math.Max(ans, stop - start);
                }
                else
                {
                    hs.Remove(s[start ++]);
                }
            }
            return ans;
    }
}