Find longest substring with unique characters

LongestSubstring.cs

using System;
using System.Collections;

namespace LongestSubstring
{
	class Program
	{
		static void Main(string[] args)
		{
			string input = "abrkaabcdefghijjxxx";
			Console.WriteLine($"Length of longest substring with unique characters: {lengthOfLongestSubstring(input)}");
			Console.ReadKey();
		}

		//This algorithm is of linear complexity O(n)
		static int lengthOfLongestSubstring(string input)
		{
			if (input is null)
				return 0;
			if (input.Length < 2)
				return input.Length;

			int longest = 1, current = 1;

			Hashtable hashtable = new Hashtable();
			hashtable.Add(input[0], null);

			for(int i = 1; i < input.Length; ++i)
			{
				if (hashtable.ContainsKey(input[i]))
				{
					if (current > longest)
					{ 
						longest = current; 
						Console.WriteLine("Longest substring found is: " + longest); 
					}

					hashtable.Clear();	//Get ready for finding another substring
					current = 0;		//Flush the counter
				}
				else // Every character is unique so far, keep adding
				{ 
					++current;
					hashtable.Add(input[i], null);
				}
			}

			return longest;
		}
	}
}