Longest Nice Substring LeetCode solution in C++

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class Solution {
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 public:
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  string longestNiceSubstring(string s) {
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    int arr1[26] = {};
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    int arr2[26] = {};
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    if (s.length() < 2) return "";
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    for (char ch : s) {
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      if (ch >= 'A' && ch <= 'Z')
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        arr1[(ch | 32) - 'a']++;
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      else
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        arr2[ch - 'a']++;
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    }
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    vector<int> index;
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    index.push_back(-1);
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    for (int i = 0; i < s.length(); i++) {
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      if ((arr1[(s[i] | 32) - 'a'] >= 1 && arr2[(s[i] | 32) - 'a'] == 0) ||
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          (arr1[(s[i] | 32) - 'a'] == 0 && arr2[(s[i] | 32) - 'a'] >= 1))
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        index.push_back(i);
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    }
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    // index.push_back(2);
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    index.push_back(s.length());
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    if (index.size() == 2) return s;
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    string minn = "";
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    for (int i = 0; i < index.size() - 1; i++) {
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      string temp = longestNiceSubstring(s.substr(index[i] + 1, index[i + 1] - index[i] - 1));
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      minn = temp.length() > minn.length() ? temp : minn;
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    }
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    return minn;
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  }
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};