Find longest substring without repeating characters

Question

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

Example:

Input:

Answer 1

def max_substring(string):
   last_substring = ''
   max_substring  = ''
   for x in string:
       k = find_index(x,last_substring)
       last_substring = last_substring[(k+1):]+x
       if len(last_substring) > len(max_substring):
               max_substring  = last_substring        
   return max_substring

def find_index(x, lst):
   k = 0
   while k