Design Authentication Manager LeetCode solution in C++

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class AuthenticationManager {
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  int ttl;
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  unordered_map<string, int> tokens;
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 public:
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  AuthenticationManager(int timeToLive) {
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    ttl = timeToLive;
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  }
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  void generate(string tokenId, int currentTime) {
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    tokens[tokenId] = currentTime + ttl;
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  }
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  void renew(string tokenId, int currentTime) {
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    auto tokenIt = tokens.find(tokenId);
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    if (tokenIt != end(tokens) && tokenIt->second > currentTime) {
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      tokenIt->second = currentTime + ttl;
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    }
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  }
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  int countUnexpiredTokens(int currentTime) {
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    int res = 0;
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    for (auto token : tokens) {
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      if (token.second > currentTime) res++;
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    }
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    return res;
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  }
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};