Map Sum Pairs LeetCode solution in C++

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class MapSum {
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 public:
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  map<string, int> mp;
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  MapSum() {}
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  void insert(string key, int val) {
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    mp[key] = val;
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  }
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  int sum(string prefix) {
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    int count = 0;
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    for (auto i : mp) {
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      int j;
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      for (j = 0; j < i.first.size() && j < prefix.size(); j++) {
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        if (i.first[j] != prefix[j]) break;
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      }
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      if (j == prefix.size()) count += i.second;
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    }
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    return count;
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  }
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};