## What is it? Find the length of the longest substring that contains no repeating characters. Classic sliding window problem solved with a hash set tracking the current window contents. ## How it works - Use `left` and `right` pointers both starting at 0 - Expand the window by moving `right` forward - If `str[right]` is already in the current window (set), shrink the window by moving `left` forward until the duplicate is removed - At each step, `maxLen = max(maxLen, right - left + 1)` - Return `maxLen` ## When to use - Any problem asking for the longest/shortest window satisfying a uniqueness or frequency constraint - Template for variable-size sliding window problems ## Key Points - O(n) time — each character enters and leaves the window at most once - O(k) space where k is the size of the character set (26 for lowercase letters, 128 for ASCII) - Use a `HashMap` (char → last index) for an optimized version that jumps `left` directly