---
title: '424. Longest Repeating Character Replacement'
description: You are given a string s and an integer k. You can choose any character of the string and change it to any other uppercase English character. You can perform this operation at most k times
sidebar:
  label: 'Longest Repeating Character Replacement'
  badge: 'Medium'
---

<Badge variant="accent">Sliding Window</Badge>

### Example 1:
- Input: `s = "ABAB", k = 2`
- Output: `4`
- Explanation: Replace the two 'A's with two 'B's or vice versa.

### Example 2:
- Input: `s = "AABABBA", k = 1`
- Output: `4`
- Explanation: Replace the one 'A' in the middle with 'B' and form "AABBBBA". The substring "BBBB" has the longest repeating letters, which is `4`. There may exists other ways to achieve this answer too.

### Constraints:

- `1 <= s.length <= 10^5`
- `s` consists of only uppercase English letters.
- `0 <= k <= s.length`

## Approach

```mermaid
flowchart TD
  S(["characterReplacement(s, k)"]) --> I["count = {}, res = 0, l = 0, maxF = 0"]
  I --> L{"more (r, c) in s?"}
  L -- no --> E(["return res"])
  L -- yes --> A["count[c] += 1; maxF = max(maxF, count[c])"]
  A --> W{"(r - l + 1) - maxF > k? — too many chars to replace"}
  W -- yes --> P["count[s[l]] -= 1; l += 1 — shrink from the left"]
  P --> W
  W -- no --> M["res = max(res, r - l + 1)"]
  M --> L
```

## Solution

```py
class Solution:
    def characterReplacement(self, s: str, k: int) -> int:
        count = {}
        res = 0

        l = 0
        maxF = 0
        for r, c in enumerate(s):
            count[c] = 1 + count.get(c, 0)
            maxF = max(maxF, count[c])

            while (r - l + 1) - maxF > k:
                count[s[l]] -= 1
                l += 1

            res = max(res, r - l + 1)
        return res
```

## Explanation

<YouTube id="gqXU1UyA8pk" title="Longest Repeating Character Replacement - Leetcode 424 - Python" />