public function getWinner()
    {
        if (!$this->isGameOver()) {
            return null;
        }

        if ($this->getScore(self::PLAYER_0) === $this->getScore(self::PLAYER_1)) {
            return self::DRAW;
        } elseif ($this->getScore(self::PLAYER_0) > $this->getScore(self::PLAYER_1)) {
            return self::PLAYER_0;
        } else {
            return self::PLAYER_1;
        }
    }