class TitleThree
{
    private $nums;
    private $count;

    public function __construct($nums, $count = 1)
    {
        if (!is_array($nums)) {
            throw new InvalidArgumentException('第一个参数必须是数组');
        }
        if (!is_numeric($count)) {
            throw new InvalidArgumentException('第二个参数必须是数字');
        }
        $this->nums = $nums;
        $this->count = $count;
    }

    /**
     * 方法一：使用数组函数统计元素出现的次数
     * @return string
     */
    public function findSingleNumber1()
    {
        $count = array_count_values($this->nums);
        $filtered = array_filter($count, function($freq) {
            return $freq === $this->count;
        });
        return implode(',', array_keys($filtered));
    }

    /**
     * 方法二：手动循环统计
     * @return string
     */
    public function findSingleNumber2()
    {
        $exist = [];
        $ignore = [];
        foreach ($this->nums as $num) {
            if (in_array($num, $ignore)) {
                continue;
            }
            if (!isset($exist[$num])) {
                $exist[$num] = 0;
            }
            $exist[$num]++;
            if ($exist[$num] > $this->count) {
                $ignore[] = $num;
                unset($exist[$num]);
            }
        }
        $res = [];
        foreach ($exist as $num => $freq) {
            if ($freq === $this->count) {
                $res[] = $num;
            }
        }
        return implode(',', $res);
    }

    /**
     * 方法三：排序后统计
     * @return string
     */
    public function findSingleNumber3()
    {
        sort($this->nums);
        $len = count($this->nums);
        $res = [];
        $currentCount = 1;
        for ($i = 1; $i < $len; $i++) {
            if ($this->nums[$i] === $this->nums[$i - 1]) {
                $currentCount++;
            } else {
                if ($currentCount === $this->count) {
                    $res[] = $this->nums[$i - 1];
                }
                $currentCount = 1;
            }
        }
        if ($currentCount === $this->count) {
            $res[] = $this->nums[$len - 1];
        }
        return implode(',', $res);
    }
}

try {
    $count = 1;
    $index = new TitleThree([2, 2, 1], $count);
    $res1 = $index->findSingleNumber1();
    echo "方法一出现 $count 次的数字结果：";
    var_dump($res1);
    $res2 = $index->findSingleNumber2();
    echo "方法二出现 $count 次的数字结果：";
    var_dump($res2);
    $res3 = $index->findSingleNumber3();
    echo "方法三出现 $count 次的数字结果：";
    var_dump($res3);

} catch (InvalidArgumentException $e) {
    echo "错误：".$e->getMessage();
}

方法一出现 1 次的数字结果：string(25) “23,45,54,22,9,5,654,66,67”
方法二出现 1 次的数字结果：string(25) “23,45,54,22,9,5,654,66,67”
方法三出现 1 次的数字结果：string(25) “5,9,22,23,45,54,66,67,654”