Greatest English Letter in Upper and Lower Case LeetCode solution in C++

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class Solution {
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 public:
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  string greatestLetter(string s) {
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    vector<int> low(26),
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        upp(26); // storing occurences of lower and upper case letters
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    string res = "";
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    for (auto it : s) // iterate over each char and mark it in respective vector
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    {
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      if (it - 'A' >= 0 && it - 'A' < 26)
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        upp[it - 'A']++;
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      else
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        low[it - 'a']++;
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    }
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    for (int i = 25; i >= 0; i--) // start from greater char
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    {
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      if (low[i] && upp[i]) // if char found in upp and low that will be the result
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      {
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        res += 'A' + i;
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        break;
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      }
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    }
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    return res;
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  }
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};