Evaluate the Bracket Pairs of a String LeetCode solution in C++

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class Solution {
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 public:
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  string evaluate(string s, vector<vector<string>> &knowledge) {
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    string ans; // resultant string
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    int n = s.size();
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    if (n < 2) return s; // because () will come in pair so, size should be more than 2
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    int sz = knowledge.size();
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    unordered_map<string, string> mp; // To reduce Runtime otherwise it will give TLE
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    for (int i = 0; i < sz; ++i) {
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      mp.insert({knowledge[i][0], knowledge[i][1]}); // Inserting {key, value} pair
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    }
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    for (int i = 0; i < n; i++) {
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      if (s[i] == '(') {
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        string key;
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        i++;
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        while (s[i] != ')') { // getting key till we get ')'
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          key += s[i++];
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        }
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        string value;
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        if (mp.find(key) != mp.end()) { // If {key, value} pair is present then
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                                        // replace (key) by it's value
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          value = mp[key];
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          ans += value;
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        } else { // otherwise replace (key) by ?
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          ans += "?";
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        }
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      } else
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        ans += s[i];
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    }
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    return ans;
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  }
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};
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