# Longest Repeating Character Replacement
- <https://leetcode.com/problems/longest-repeating-character-replacement/>
- Maintaining the properties of the window.
```python
counter = Counter()
w = max_len = start = max_count = 0
for c in s:
w += 1
counter[c] += 1
if max_count < counter[c]:
max_count = counter[c]
if w - max_count > k:
counter[s[start]] -= 1
start += 1
w -= 1
else:
max_len = w if max_len < w else max_len
return max_len
```