一个强悍的算24点游戏的PHP程序(递归函数的妙用)

研究递归函数 找到的一个经典的例子 

<?php

set_time_limit(0); 
$values = array(5, 5, 5, 1); 
$result = 24;

$list = array();

echo "<pre>"; 
makeValue($values); 
print_r($list);

function makeValue($values, $set=array()) 
{ 
	$words = array("+", "-", "*", "/"); 
	if(sizeof($values)==1) 
	{ 
		$set[] = array_shift($values); 
		return makeSpecial($set); 
	} 
	
	foreach($values as $key=>$value) 
	{ 
		$tmpValues = $values; 
		unset($tmpValues[$key]); 
		foreach($words as $word) 
		{ 
			makeValue($tmpValues, array_merge($set, array($value, $word))); 
		} 
	} 
} 

function makeSpecial($set) 
{ 
	$size = sizeof($set);

	if($size<=3 || !in_array("/", $set) && !in_array("*", $set)) 
	{ 
		return makeResult($set); 
	}

	for($len=3; $len<$size-1; $len+=2) 
	{ 
		for($start=0; $start<$size-1; $start+=2) 
		{ 
			if(!($set[$start-1]=="*" || $set[$start-1]=="/" || $set[$start+$len]=="*" || $set[$start+$len]=="/")) 
				continue; 
			$subSet = array_slice($set, $start, $len); 
			if(!in_array("+", $subSet) && !in_array("-", $subSet)) 
				continue; 
			$tmpSet = $set; 
			array_splice($tmpSet, $start, $len-1); 
			$tmpSet[$start] = "(".implode("", $subSet).")"; 
			makeSpecial($tmpSet); 
		} 
	} 
}

function makeResult($set) 
{ 
	global $result, $list; 
	$str = implode("", $set); 
	@eval("\$num=$str;"); 
	if($num==$result && !in_array($str, $list)) 
	$list[] = $str; 
}

?>