二分查找及其變種

上次寫個二分查找，錯誤百出，唉，還真如編程珠璣裏面說的，寫二分查找的人 90%都不會一次成功。深深的鄙視一下自己，貼一下二分的代碼，以及二分查找的變種代碼

 

//二分查找
function binary _search($search, $data_arr, $lower, $upper)
{
    if($lower > $upper)
    {
        return false;
    }

    $mid = intval( ($lower + $upper) / 2 );

    if($search == $data_arr[$mid])
    {
        return $mid;
    }elseif($search < $data_arr[$mid])
    {
        $upper = $mid - 1;
    }else
    {
        $lower = $mid + 1;
    }

    return search($search, $data_arr, $lower, $upper);
}

/*
 * 二分查找變種，查找$search第一次出現的位置 ,對於傳值，$lower=-1  $upper=count($data_arr)
 *
 * array(0,2,3,7,8,8,8,9,10)
 * binary_search_begin(8, ....)
 * 返回值4
 */
function binary_search_begin($search, $data_arr, $lower, $upper)
{
    if( ($lower + 1) == $upper )
    {
        return ($data_arr[$upper] == $search)? $upper : -1;
    }

    $mid = intval( ($lower + $upper) / 2 );

    if( $data_arr[$mid] < $search )
    {
        $lower = $mid;
    }else
    {
        $upper = $mid;
    }

    return binary_search_begin($search, $data_arr, $lower, $upper);
}

/*
 * 二分查找變種，查找$search最後一次出現的位置 ,對於傳值，$lower=-1  $upper=count($data_arr)
 * array(0,2,3,7,8,8,8,9,10)
 * binary_search_begin(8, ....)
 * 返回值6
 */
function binary_search_end($search, $data_arr, $lower, $upper)
{
    if( ($lower + 1) == $upper )
    {
        return ($data_arr[$lower] == $search)? $lower : -1;
    }

    $mid = intval( ($lower + $upper) / 2 );

    if( $data_arr[$mid] > $search )
    {
        $upper = $mid;
    }else
    {
        $lower = $mid;
    }

    return binary_search_end($search, $data_arr, $lower, $upper);
}

 

其迭代方法的實現爲：

 

//二分查找
function binary_search($search, $data_arr, $lower, $upper)
{
    while($lower <= $upper)
    {
        $mid = intval( ($lower + $upper) / 2 );

        if($data_arr[$mid] > $search)
        {
            $upper = $mid - 1;
        }elseif($data_arr[$mid] < $search)
        {
            $lower = $mid + 1;
        }else
        {
            return $mid;
        }
    }

    return false;
}

/*
 * 二分查找變種，查找$search第一次出現的位置,對於傳值，$lower=-1  $upper=count($data_arr)
 * array(0,2,3,7,8,8,8,9,10)
 * binary_search_begin(8, ....)
 * 返回值4
 */
function binary_search_begin($search, $data_arr, $lower, $upper)
{
    while($lower + 1 != $upper)
    {
        $mid = intval( ($lower + $upper) / 2 );

        if($data_arr[$mid] < $search)
        {
            $lower = $mid;
        }else
        {
            $upper = $mid;
        }

        $p = $upper;

        if($data_arr[$p] != $search)
        {
            $p = false;
        }
    }

    return $p;
}

/*
 * 二分查找變種，查找$search最後一次出現的位置，對於傳值，$lower=-1  $upper=count($data_arr)
 * array(0,2,3,7,8,8,8,9,10)
 * binary_search_begin(8, ....)
 * 返回值6
 */
function binary_search_end($search, $data_arr, $lower, $upper)
{
    while($lower + 1 != $upper)
    {
        $mid = intval( ($lower + $upper) / 2 );

        if($data_arr[$mid] > $search)
        {
            $upper = $mid;
        }else
        {
            $lower = $mid;
        }

        $p = $lower;
        if($data_arr[$p] != $search)
        {
            $p = false;
        }
    }

    return $p;

}