Longest Repeating Character Replacement

Question

## Problem

Given a string and integer `k`, find the longest substring of one repeating letter achievable by replacing at most `k` characters.

## Example
```
s="AABABBA", k=1 -> 4
```

## Constraints
- 1 ≤ |s| ≤ 10^5

## Approach
Sliding window tracking the most frequent char count; window valid while (len - maxFreq) ≤ k.