Longest Repeating Character Replacement LeetCode solution in C++

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class Solution {
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 public:
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  int characterReplacement(string s, int k) {
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    int n = s.length();
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    if (n == k) return n;
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    if (n == 1) return 1;
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    int res = 0;
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    int maxCnt = 0;
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    unordered_map<char, int> mp;
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    for (int l = 0, r = 0; r < n; r++) {
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      mp[s[r]]++;
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      maxCnt = max(maxCnt, mp[s[r]]);
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      while (r - l + 1 - maxCnt > k) {
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        mp[s[l]]--;
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        l++;
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      }
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      res = max(r - l + 1, res);
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    }
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    return res;
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  }
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};