Find a longest substring with a sliding window

Keep a left boundary and the most recent position of each character. When a character repeats inside the current window, move the left boundary past its previous position. Never move it backwards. Each character is visited once: expected O(n) time and O(u) space for u distinct characters. Sum-based windows need additional assumptions: negative values can break the usual expand-and-shrink rule.

def longest_unique(text):
    last = {}
    left = best = 0
    for right, char in enumerate(text):
        left = max(left, last.get(char, -1) + 1)
        last[char] = right
        best = max(best, right - left + 1)
    return best

assert longest_unique("abba") == 2
assert longest_unique("") == 0

These examples use Python 3 and include assertions you can run locally.