# 424. Longest Repeating Character Replacement

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.

Return *the length of the longest substring containing the same letter you can get after performing the above operations*.

**Example 1:**

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

**Example 2:**

```python
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<sup>5</sup>`
    
* `s` consists of only uppercase English letters.
    
* `0 <= k <= s.length`
    

Solution:

```python
class Solution:
    def characterReplacement(self, s: str, k: int) -> int:
        # hash map to count the item in window
        count = {}
        l = 0
        maxf = 0
        # iterate through string
        for r in range(len(s)):
            # count
            count[s[r]] = 1 + count.get(s[r], 0)
            # find the max lenth of window
            maxf = max(maxf, count[s[r]])
            # check for invalid window
            if (r - l + 1) - maxf > k:
                count[s[l]] -= 1
                l += 1
        # return the max window substring
        return (r - l + 1)
```