String problem created by vetional - https://repl.it/MRdh/1

main.py

# Given a string, find the longest substring without repeating characters.
from collections import defaultdict

def solution(s):
  if not s:
    return None
  
  tmp = ""
  longest = ""
  dict = defaultdict(int)
  for i,c in enumerate(s):
    if c not in dict:
      dict[c] = 1
      tmp += c
    else:
      if len(tmp) > len(longest):
        longest = tmp
      dict.clear()
      tmp = c
      dict[c] = 1
  
  if len(longest) > len(tmp): 
    return longest
  else: 
    return tmp
  
print solution("abccdefghhajajaajkluuioopwq")