# 424. Longest Repeating Character Replacement

Given a string `s` that consists of only uppercase English letters, you can perform at most `k` operations on that string.

In one operation, you can choose **any** character of the string and change it to any other uppercase English character.

Find the length of the longest sub-string containing all repeating letters you can get after performing the above operations.

**Note:**\
Both the string's length and k will not exceed 104.

**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.
```

```cpp
// Sliding window
int characterReplacement(string s, int k) { // time: O(n); space: O(1)
    int start = 0, end = 0, maxCount = 0, d = 0;
    vector<int> record(128, 0);
    while (end < s.length()) {
        maxCount = max(maxCount, ++record[s[end++]]);
        while (end - start - maxCount > k) {
            --record[s[start++]];
        }
        d = max(d, end - start);
    }
    return d;
}
```

```cpp
// Sliding Window for only uppercase character input
int characterReplacement(string s, int k) { // time: O(n); space: O(1)
    int begin = 0, end = 0, maxCnt = 0, res = 0;
    vector<int> record(26, 0);
    while (end < s.length()) {
        maxCnt = max(maxCnt, ++record[s[end++] - 'A']);
        // (end - begin - maxCnt) is # of the replacement operation required
        while (end - begin - maxCnt > k) --record[s[begin++] - 'A'];
        res = max(res, end - begin);
    }
    return res;
}
```