Longest Substring Without Repeating Characters

A quick note: this problem is backed by a real Omnissa onsite interview report that directly named Longest Substring Without Repeating Characters. The report did not include an exact function interface, examples, constraints, or character-set rules. The core task match is about 95%.
Given a string s, return the length of the longest substring that contains no repeated characters.
Interview Follow-up
This was part of a rapid-fire three-problem round in which the candidate presented brute-force, improved, and optimal approaches and wrote working code for each problem.

Function
lengthOfLongestSubstring(s: String) → int

Examples
Example 1
s = "abcabcbb"
return = 3
The longest substrings without repeated characters include "abc", with length 3.
Example 2
s = "bbbbb"
return = 1
Every valid substring without repeats has length 1.

The trick is a sliding window with a hash map from character to its last seen index. Walk the string with a right pointer. When the current character was seen at an index inside the window, jump the left pointer to that index plus one. Update the best length each step. That's O(n) time. The brute force checks every substring with a set, which is O(n^2) or O(n^3). The middle approach is a window that shrinks one step at a time with a set. The common pitfall is moving the left pointer backward. Always take max(left, lastSeen + 1), or stale indexes will break your answer on input like "abba". Also handle the empty string and a single character. The hinted dynamic-programming label is loose. This is a window problem. If your mind goes blank mid-assessment, StealthCoder can supply the clean version as a hedge, but know the three tiers cold.

StealthCoder is the hedge for the one pattern you didn't drill. It runs invisibly during the screen share.