php 二分查找法算法详解
一、概念：二分查找又称折半查找，优点是比较次数少，查找速度快，平均性能好；其缺点是要求待查表为有序表，且插入删除困难。因此，折半查找方法适用于不经常变动而查找频繁的有序列表。首先，假设表中元素是按升序排列，将表中间位置记录的关键字与查找关键字比较，如果两者相等，则查找成功；否则利用中间位置记录将表分成前、后两个子表，如果中间位置记录的关键字大于查找关键字，则进一步查找前一子表，否则进一步查找后一子表。重复以上过程，直到找到满足条件的记录，使查找成功，或直到子表不存在为止，此时查找不成功。
二、代码：对于无序数组用以下方法即可。
header("Content-type:text/html;charset='utf-8'");
function twosearchmethod($arr,$val,$left,$right){
if($left>$right){
echo "找不到该数值";
return ;
}
$middle=round(($left+$right)/2);
if($arr[$middle]>$val){
twosearchmethod($arr, $val, $left, $middle-1);
}elseif($arr[$middle]<$val){
twosearchmethod($arr, $val, $middle+1, $right);
}else{
echo $middle;
}
}
$arr=array(1,9,3,4,5,6,7);
sort($arr);
print_r($arr);
echo "<br/>";
$val=1;
twosearchmethod($arr, $val, 0, 6);
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