Longest Repeating Character Replacement
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# LeetCode #424: Longest Repeating Character Replacement (C/C++).

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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 <= 105
• 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.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 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; } };