using System;

using System.Collections.Generic;   

using System.Text;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        string longest = longestSubstring("pwwkewnwpkrsn");

        Console.WriteLine("--longest--");

        Console.WriteLine(longest);

    }

    public static string longestSubstring(string str){

        List<string> strings = new List<string>();

        string substring = "";

        for(int i = 0; i< str.Length; i++){

            char c = str[i];

            int index = substring.IndexOf(c);

            if(index == -1){

                substring += c.ToString();  

            }else{

                strings.Add(substring); 

                substring = substring.Substring(index + 1) + c.ToString();

            }

        }

        strings.Add(substring);

        string longest = "";

        foreach(var s in strings){

            if(s.Length > longest.Length){

                longest = s;    

            }

            Console.WriteLine(s);   

        }

        return longest;

    }

}