# 1128.Number of Equivalent Domino Pairs

**1128.Number of Equivalent Domino Pairs**

难度:Easy

> 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].

```
Example 1:

Input: dominoes = [[1,2],[2,1],[3,4],[5,6]]
Output: 1
 

Constraints:

1 <= dominoes.length <= 40000
1 <= dominoes[i][j] <= 9
```

可以先按大小排个序，然后建立一个map，统计次数大于1的元素个数，每个元素的对数为C(2,n)

```
class Solution {
public:
    int numEquivDominoPairs(vector<vector<int>>& dominoes) {
        int res=0;
        if(dominoes.size()<2) return res;
        for(auto &v: dominoes)
            if(v[0]>v[1]) swap(v[0],v[1]);
        map<vector<int>, int> domin_map;
        for(auto v:dominoes)
            domin_map[v]++;
        for(auto v:domin_map)
            if(v.second >1) res+= v.second * (v.second -1);
        return res/2;
        
            
        
    }
};
```

> 执行用时 :68 ms, 在所有 C++ 提交中击败了76.11%的用户\
> 内存消耗 :23.5 MB, 在所有 C++ 提交中击败了100.00%的用户