License Key Formatting LeetCode solution in C++

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class Solution {
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 public:
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  string licenseKeyFormatting(string S, int K) {
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    stack<char> sChars;
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    for (auto ch : S) {
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      if (ch != '-') sChars.push(ch);
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    }
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    string sRet;
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    int cur = 0;
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    while (!sChars.empty()) {
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      sRet += toupper(sChars.top());
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      sChars.pop();
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      cur++;
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      if (cur == K && !sChars.empty()) {
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        sRet += '-';
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        cur = 0;
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      }
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    }
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    reverse(sRet.begin(), sRet.end());
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    return sRet;
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  }
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};