Increasing Decreasing String LeetCode solution in C++

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class Solution {
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 public:
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  string sortString(string s) {
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    int count[26] = {0};
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    for (int i = 0; i < s.size(); i++) {
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      char ch = s[i];
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      count[ch - 'a']++;
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    }
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    string res;
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    while (res.size() != s.size()) {
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      for (int i = 0; i < 26; i++) {
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        if (count[i] > 0) {
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          char ch = i + 'a';
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          res += ch;
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          count[i]--;
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        }
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      }
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      for (int i = 25; i >= 0; i--) {
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        if (count[i] > 0) {
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          char ch = i + 'a';
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          res += ch;
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          count[i]--;
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        }
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      }
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    }
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    return res;
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  }
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};