Longest Substring Without Repeating - Implementation

Here is the implementation:

function lengthOfLongestSubstring(s):
    seen = empty set
    left = 0
    maxLen = 0

    for right from 0 to s.length - 1:
        while s[right] in seen:
            seen.remove(s[left])
            left += 1

        seen.add(s[right])
        maxLen = max(maxLen, right - left + 1)

    return maxLen

$O(n)$ time (each character added/removed once), $O(min(n, alphabet))$ space for the set.