Number of Equivalent Domino Pairs
<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>
Array; Hash Table; Counting
impl Solution {
    pub fn num_equiv_domino_pairs(dominoes: Vec<Vec<i32>>) -> i32 {
        let mut cnt = [0i32; 100];
        let mut ans = 0;

        for d in dominoes {
            let a = d[0] as usize;
            let b = d[1] as usize;
            let key = if a < b { a * 10 + b } else { b * 10 + a };
            ans += cnt[key];
            cnt[key] += 1;
        }

        ans
    }
}
