**source:**https://leetcode.com/problems/longest-repeating-character-replacement**C/C++**

**Solution to LeetCode**problem

**424**.

**Longest Repeating Character Replacement**.

## Problem

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*.

## Examples

**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.

## Constraints

`1 <= s.length <= 10`

^{5}`s`

consists of only uppercase English letters.`0 <= k <= s.length`

## Solution

We start with a `window size = 1`

, and we start growing the window.

- Using a hash table.
- We increase the count for the added character.
- We track max frequency (most repeated character)
- if the
`window size - the max freq`

is`>`

than the allowed elements to change- We decrease the count of the character.
- We increase the initial index for our final string.

- We update the new max size of the final string.

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class Solution {
public:
int characterReplacement(string s, int k) {
int j = 0;
int maxfq = 0;
int result = 0;
unordered_map<char, int> mp;
for (int i=0; i<s.size(); i+=1) {
mp[s[i]] += 1;
maxfq = max(maxfq, mp[s[i]]);
if (i - j + 1 - maxfq > k) {
mp[s[j]] -= 1;
j += 1;
}
result = max(res, i-j+1);
}
return result;
}
};