class Solution {
public:
    int characterReplacement(string s, int k) {
        int n = s.size(), l=0, r=0;
        unordered_map<char, int>window;
        window[s[l]]++;
        
        int ans = 1;
        while(l<=r && r<n){
            int maxFreq = 0;
            for(auto it: window){
                int fr = it.second;
                maxFreq = max(maxFreq, fr);
            }
            int changesReq = r-l+1 - maxFreq;
            if(changesReq <= k){
                ans = max(ans, r-l+1);
                r++;
                window[s[r]]++;
            }
            else{
                window[s[l]]--;
                if(window[s[l]] <= 0) window.erase(s[l]);
                l++;
            }
        }
        return ans;
    }
};