public function getWordsByComplexity($string)
    {
        $words = $this->getWords($string);
        $analysis = new FrequencyAnalysis($words);
        $sortedWords = $analysis->getKeyValuesByFrequency();
        $wordsByFrequency = array_unique(array_keys($sortedWords));

        usort($wordsByFrequency, function ($a, $b) {
            return strlen($b) - strlen($a);
        });

        return $wordsByFrequency;
    }