输入任意4个数字，然后对其进行+-*/组合，所得数学表达式值等于24

算法思路：把每一个数字看做一个独立的数学表达式，表达式之间加上标点符号组合成新表达式，一共组合4次，表达式之间的所有组合可以通过递归来实现.
代码如下：

 
<?php 
/** 
* A 24 maker 
* @version 1.0.0 
* @author laruence<laruence at yahoo.com.cn> 
* @copyright (c) 2009 http://www.laruence.com 
*/ 
class TwentyFourCal { 
public $needle = 24; 
public $precision = '1e-6'; 
function TwentyFourCal() { 
} 
private function notice($mesg) { 
var_dump($mesg); 
} 
/** 
* 取得用户输入方法 
*/ 
public function calculate($operants = array()) { 
try { 
$this->search($operants, 4); 
} catch (Exception $e) { 
$this->notice($e->getMessage()); 
return; 
} 
$this->notice('can\'t compute!'); 
return; 
} 
/** 
* 求24点算法PHP实现 
*/ 
private function search($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() { 
} 
} 
/* demo */ 
$tf = new TwentyFourCal(); 
$tf->calculate( array(4,8,8,8) ); 
?>