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

<p>Return <em>the number of pairs </em><code>(i, j)</code><em> for which </em><code>0 &lt;= i &lt; j &lt; dominoes.length</code><em>, and </em><code>dominoes[i]</code><em> is <strong>equivalent to</strong> </em><code>dominoes[j]</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> dominoes = [[1,2],[2,1],[3,4],[5,6]]
<strong>Output:</strong> 1
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> dominoes = [[1,2],[1,2],[1,1],[1,2],[2,2]]
<strong>Output:</strong> 3
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= dominoes.length &lt;= 4 * 10<sup>4</sup></code></li>
	<li><code>dominoes[i].length == 2</code></li>
	<li><code>1 &lt;= dominoes[i][j] &lt;= 9</code></li>
</ul>

class Solution {
public:
        void sortDomino(vector<int> &domino) {
    if (domino[0] > domino[1]) {
        int temp = domino[0];
        domino[0] = domino[1];
        domino[1] = temp;
    }
}

int numEquivDominoPairs(vector<vector<int> >& dominoes) {
    unordered_map<string, int> map;
    int count = 0;
    for (int i = 0; i < dominoes.size(); i++) {
        sortDomino(dominoes[i]);
        string key = to_string(dominoes[i][0]) + to_string(dominoes[i][1]);
        if (map.find(key) != map.end()) {
            count += map[key];
            map[key]++;
        }
        else {
            map[key] = 1;
        }
    }
    return count;
}
};
27000