PHP历理 做一个计算24点的功能完善版

<?php
/* demo */
$tf = new TwentyFourCal();
$tf->calculate( array(5,5,5,5) );
$tf->calculate( array(2,4,10,10) );
$tf->calculate( array(4,4,10,10) );
$tf->calculate( array(1,2,7,7) );
$tf->calculate( array(4,4,4,4) );
$tf->calculate( array(1,2,1,2) );

class TwentyFourCal {
    public  $needle = 24;
    public  $precision = '1e-6';
    
    private function notice($mesg) {
        echo $mesg.'<br>';
    }
 
    /**
     * 取得用户输入方法
     */
    public function calculate($operants = array()) {
        try {
            $this->search($operants, 4);
        } catch (Exception $e) {
            $this->notice($e->getMessage());
            return;
        }
        $this->notice('计算不出24！');
        return;
    }
 
    /**
     * 求24点算法PHP实现
     */
    private function search($expressions, $level) {
        if ($level == 1) {
            $result = 'return ' . $expressions[0] . ';';
            if ( abs(eval($result) - $this->needle) <= $this->precision) {
                throw new Exception($expressions[0]);
            }
        }
        for ($i=0;$i<$level;$i++) {
            for ($j=$i+1;$j<$level;$j++) {
                $expLeft  = $expressions[$i];
                $expRight = $expressions[$j];
                $expressions[$j] = $expressions[$level - 1];
 
                $expressions[$i] = '(' . $expLeft . ' + ' . $expRight . ')';
                $this->search($expressions, $level - 1);
 
                $expressions[$i] = '(' . $expLeft . ' * ' . $expRight . ')';
                $this->search($expressions, $level - 1);
 
                $expressions[$i] = '(' . $expLeft . ' - ' . $expRight . ')';
                $this->search($expressions, $level - 1);
 
                $expressions[$i] = '(' . $expRight . ' - ' . $expLeft . ')';
                $this->search($expressions, $level - 1);
                
                if ($expLeft != 0) {
                    $expressions[$i] = '(' . $expRight . ' / ' . $expLeft . ')';
                    $this->search($expressions, $level - 1);
                }
                
                if ($expRight != 0) {
                    $expressions[$i] = '(' . $expLeft . ' / ' . $expRight . ')';
                    $this->search($expressions, $level - 1);
                }
                $expressions[$i] = $expLeft;
                $expressions[$j] = $expRight;
            }
        }
        return false;
    }
 
    function __destruct() {
    }
}
?>

效果图：

((5 * 5) - (5 / 5))
((2 + (4 / 10)) * 10)
(((10 * 10) - 4) / 4)
(((7 * 7) - 1) / 2)
((4 + 4) + (4 * 4))
计算不出24！