Longest substring without repeating

Detailed answer

Sliding window with a map/set of last seen indices. Expand right; when duplicate found, move left past previous occurrence. Track max window length.

Code example

def length_of_longest_substring(s: str) -> int:
    last = {}
    left = best = 0
    for right, ch in enumerate(s):
        if ch in last and last[ch] >= left:
            left = last[ch] + 1
        last[ch] = right
        best = max(best, right - left + 1)
    return best

Practice code · python (view only · no execution)

def length_of_longest_substring(s: str) -> int:
    last = {}
    left = best = 0
    for right, ch in enumerate(s):
        if ch in last and last[ch] >= left:
            left = last[ch] + 1
        last[ch] = right
        best = max(best, right - left + 1)
    return best