using System;

using System.Collections.Generic;

using System.Text;

public class Program

{

​

    public static void Main()

    {

        string s="abcabcbb";

        string res= LongestSubStringWithoutRepeating(s);

        Console.Write(res);

    }

    public static string LongestSubStringWithoutRepeating(string s)

    {

        int[] longest={0,1};

        int startIdx=0;

        Dictionary<char,int> ch=new Dictionary<char,int>();

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

            char c=s[i];

        if(ch.ContainsKey(c)) //abcabcbb.. a

        {

               startIdx=Math.Max(startIdx,ch[c]+1); //startidx=3

        }

            if(longest[1]-longest[0]<i+1-startIdx){ //1,4 =3<2 | 2,5 |7-3=4  == 3,7

                longest=new int[]{startIdx,i+1};

            }

            ch[c]=i; //a-0,b-1,c-2 //b.7

         }

        return s.Substring(longest[0],longest[1]-longest[0]);

    }

}

​