Number of Equivalent Domino Pairs LeetCode solution in C++

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class Solution {
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 public:
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  int numEquivDominoPairs(vector<vector<int>> &dominoes) {
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    map<pair<int, int>, int> m;
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    int ans = 0;
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    for (auto &d : dominoes) {
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      int a = min(d[0], d[1]), b = max(d[0], d[1]);
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      m[{a, b}]++;
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    }
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    for (auto &p : m) {
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      int n = p.second;
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      ans += ((n - 1) * n) / 2;
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    }
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    return ans;
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  }
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};
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