def longest_unique_substring(s: str) -> str: """ Given a string , return the longest unique substring that occurs within . A unique substring is a substring within which DOES NOT have any repeating...

def longest_unique_substring(s: str) -> str:
"""
Given a string
, return the longest unique substring that occurs within

.


A unique substring is a substring within

which DOES NOT have any
repeating characters. As an example, "xd" is unique but "xxd" is not.


If there are two equal length unique substrings within
, return the one
that starts first (i.e., begins at a smaller index).


tips:
In order to get your function to run fast, consider using a dictionary to
store the indexes of previously seen characters, from there, you can
follow a set of rules based on each new character you see to determine
the length of the longest unique substring seen so far.


>>> longest_unique_substring('aab')
'ab'