# Longest Repeating Character Replacement Problem

## Description

LeetCode Problem 424.

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:

``````1
2
3
Input: s = "ABAB", k = 2
Output: 4
Explanation: Replace the two 'A's with two 'B's or vice versa.
``````

Example 2:

``````1
2
3
4
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.
``````

Constraints:

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

## Sample C++ Code

``````1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
class Solution {
public:
int characterReplacement(string s, int k) {
int len = s.size();

int maxlen = 0;
for (char c = 'A'; c <= 'Z'; c ++) {
int left = 0;
int replaced = 0;
for (int right = 0; right < len; right ++) {
if (s[right] != c) {
replaced ++;
}
if (replaced <= k) {
maxlen = max(maxlen, right - left + 1);
} else {
while (left < right && replaced > k) {
if (s[left] != c)
replaced --;
left ++;
}
}
}
}
return maxlen;
}
};
``````