Find longest substring without repeating characters

Question

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

Example:

Input:

Answer 1

^{I was asked the same question in an interview.}

I have written Python3 code, to find the first occurrence of the substring with all distinct chars. In my implementations, I start with index = 0 and iterate over the input string. While iterating used a Python dict seems to store indexes of chars in input-string those has been visited in the iteration.

In iteration, if char c, does not find in current substring – raise KeyError exception

if c found to be a duplicate char in the current substring (as c previously appeared during iteration – named that index last_seen) start a new substring

def lds(string: str) -> str:
    """ returns first longest distinct substring in input `string` """
    seens = {}
    start, end, curt_start = 0, 0, 0
    for curt_end, c in enumerate(string):
        try:
            last_seen = seens[c]
            if last_seen < curt_start:
                raise KeyError(f"{c!r} not found in {string[curt_start: curt_end]!r}")
            if end - start <  curt_end - curt_start:
                start, end = curt_start, curt_end
            curt_start = last_seen + 1
        except KeyError:
            pass
        seens[c] = curt_end
    else: 
        # case when the longest substring is suffix of the string, here curt_end
        # do not point to a repeating char hance included in the substring
        if string and end - start <  curt_end - curt_start + 1:
            start, end = curt_start, curt_end + 1
    return string[start: end]