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) );
?>