1128. Number of Equivalent Domino Pairs

Given a list of dominoes, dominoes[i] = [a, b] is equivalent to dominoes[j] = [c, d] if and only if either (a == c and b == d), or (a == d and b == c) - that is, one domino can be rotated to be equal to another domino.

Return the number of pairs (i, j) for which 0 <= i < j < dominoes.length, and dominoes[i] is equivalent to dominoes[j].

class Solution {
public:
    int numEquivDominoPairs(vector<vector<int>>& dominoes) {
        map<pair<int,int>,int> mp;
        int res = 0;
        for(auto& d : dominoes) {
            pair<int,int> p{min(d[0],d[1]),max(d[0],d[1])};
            res += mp[p]++;
        }
        return res;
    }
};