X of a Kind in a Deck of Cards
<p>You are given an integer array <code>deck</code> where <code>deck[i]</code> represents the number written on the <code>i<sup>th</sup></code> card.</p>

<p>Partition the cards into <strong>one or more groups</strong> such that:</p>

<ul>
	<li>Each group has <strong>exactly</strong> <code>x</code> cards where <code>x &gt; 1</code>, and</li>
	<li>All the cards in one group have the same integer written on them.</li>
</ul>

<p>Return <code>true</code><em> if such partition is possible, or </em><code>false</code><em> otherwise</em>.</p>

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

<pre>
<strong>Input:</strong> deck = [1,2,3,4,4,3,2,1]
<strong>Output:</strong> true
<strong>Explanation</strong>: Possible partition [1,1],[2,2],[3,3],[4,4].
</pre>

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

<pre>
<strong>Input:</strong> deck = [1,1,1,2,2,2,3,3]
<strong>Output:</strong> false
<strong>Explanation</strong>: No possible partition.
</pre>

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

<ul>
	<li><code>1 &lt;= deck.length &lt;= 10<sup>4</sup></code></li>
	<li><code>0 &lt;= deck[i] &lt; 10<sup>4</sup></code></li>
</ul>
Array; Hash Table; Math; Counting; Number Theory
class Solution {
    public boolean hasGroupsSizeX(int[] deck) {
        Map<Integer, Integer> cnt = new HashMap<>();
        for (int x : deck) {
            cnt.merge(x, 1, Integer::sum);
        }
        int g = cnt.get(deck[0]);
        for (int x : cnt.values()) {
            g = gcd(g, x);
        }
        return g >= 2;
    }

    private int gcd(int a, int b) {
        return b == 0 ? a : gcd(b, a % b);
    }
}