Find Lucky Integer in an Array
<p>Given an array of integers <code>arr</code>, a <strong>lucky integer</strong> is an integer that has a frequency in the array equal to its value.</p>

<p>Return <em>the largest <strong>lucky integer</strong> in the array</em>. If there is no <strong>lucky integer</strong> return <code>-1</code>.</p>

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

<pre>
<strong>Input:</strong> arr = [2,2,3,4]
<strong>Output:</strong> 2
<strong>Explanation:</strong> The only lucky number in the array is 2 because frequency[2] == 2.
</pre>

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

<pre>
<strong>Input:</strong> arr = [1,2,2,3,3,3]
<strong>Output:</strong> 3
<strong>Explanation:</strong> 1, 2 and 3 are all lucky numbers, return the largest of them.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> arr = [2,2,2,3,3]
<strong>Output:</strong> -1
<strong>Explanation:</strong> There are no lucky numbers in the array.
</pre>

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

<ul>
	<li><code>1 &lt;= arr.length &lt;= 500</code></li>
	<li><code>1 &lt;= arr[i] &lt;= 500</code></li>
</ul>
Array; Hash Table; Counting
class Solution {
    /**
     * @param Integer[] $arr
     * @return Integer
     */
    function findLucky($arr) {
        $cnt = array_fill(0, 501, 0);
        foreach ($arr as $x) {
            $cnt[$x]++;
        }
        for ($x = 500; $x > 0; $x--) {
            if ($cnt[$x] === $x) {
                return $x;
            }
        }
        return -1;
    }
}