Problem Statement
Given a string s, find the length of the longest substring without repeating characters.
Approach
Use a sliding window (left, right) and a Set/Map to track characters in the window. Expand right. If char is in the set, contract left until char is removed. Update max length at each step.
Time & Space Complexity
Time complexity is O(n). Space complexity is O(min(n, m)) where m is char set size.