Longest Substring Without Repeating Characters

Question

## Problem

Find the length of the longest substring without repeating characters.

## Example
```
"abcabcbb" -> 3
```

## Constraints
- 0 ≤ |s| ≤ 5 × 10^4

## Approach
Sliding window + last-seen map. Zoom favors clean, fast LC easy-medium.