06：归并排序

算法原理：

PHP代码：

function mergeSort($arr) {
    $count = count($arr);
    if ($count < 2) {
        return $arr;
    }
    $middle = floor($count/2);
    $left = array_slice($arr, 0, $middle);
    $right = array_slice($arr, $middle);
    $left = mergeSort($left);
    $right = mergeSort($right);
    return merge($left, $right);
}
function merge($arr1, $arr2) {
    $result = [];
    while (count($arr1) > 0 && count($arr2) > 0) {
        if ($arr1[0] > $arr2[0]) {
            $result[] = array_shift($arr2);
        } else {
            $result[] = array_shift($arr1);
        }
    }
    if(count($arr1)) {
        $result = array_merge($result, $arr1);
    }
    if(count($arr2)) {
        $result = array_merge($result, $arr2);
    }
    return $result;
}