04：希尔排序

算法原理：

PHP代码：

function shellSort($arr) {
    $count = count($arr);
    if ($count < 2) {
        return = $arr;
    }
    for ($gap=floor($count/2); $gap > 0; $gap = floor($gap/2)) { 
        for ($i=$gap; $i < $count; $i++) { 
            $j = $i - $agp;
            $tmp = $arr[$i];
            while ($j >= 0 && $arr[$j] > $tmp) {
                $arr[$j + $gap] = $arr[$j];
                $j = $j - $gap;
            }
            $arr[$j + $gap] = $tmp;
        }
    }
    return $arr;
}