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#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

// Function to calculate the length of the longest substring
// with at most k replacements
int characterReplacement(string s, int k) {
    // Frequency array to count occurrences of each character
    vector<int> freq(26, 0);

    int max_len = 0;  // Stores the maximum length of the substring
    int max_freq = 0; // Stores the frequency of the most common character in the window
    int start = 0;    // Left pointer for the sliding window

    // Iterate over the string with the 'end' pointer
    for (int end = 0; end < s.size(); ++end) {
        // Increment the frequency of the current character
        freq[s[end] - 'A']++;

        // Update the max frequency in the current window
        max_freq = max(max_freq, freq[s[end] - 'A']);

        // If the number of replacements required exceeds 'k', move the 'start' pointer
        while (end - start + 1 - max_freq > k) {
            freq[s[start] - 'A']--; // Decrease the frequency of the character being removed
            start++; // Move the window to the right
        }

        // Update the maximum length of the valid window
        max_len = max(max_len, end - start + 1);
    }

    return max_len;
}

int main() {
    // Test case 1
    string s1 = "ABAB";
    int k1 = 2;
    cout << "Test Case 1: " << characterReplacement(s1, k1) << endl; // Expected output: 4

    // Test case 2
    string s2 = "AABABBA";
    int k2 = 1;
    cout << "Test Case 2: " << characterReplacement(s2, k2) << endl; // Expected output: 4

    return 0;
}