Merge Similar Items LeetCode solution in C++

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class Solution {
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 public:
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  vector<vector<int>> mergeSimilarItems(vector<vector<int>> &items1, vector<vector<int>> &items2) {
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    map<int, int> m;
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    for (auto v : items1) m[v[0]] += v[1];
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    for (auto v : items2) m[v[0]] += v[1];
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    vector<vector<int>> ans;
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    for (auto i : m) ans.push_back({i.first, i.second});
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    return ans;
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  }
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};