PHP 归并算法-分而治之

<?php
$arr = [5,12,3,55,3,9,10,3,23,123,4,9];

/**
 * [branch 分]
 * @param  array  $arr [description]
 * @return [type]      [description]
 */
function branch( array $arr):array{
    // 切割成粒度为1的数组返回
    if(count($arr) == 1 ){
        return $arr;
    }
    $s = intval(count($arr) /2);
    $left = array_slice($arr,0,$s);
    $right = array_slice($arr,$s);
    
    $left = branch($left);
    $right = branch($right);
    // left right两个数组进行排序合并
    $arr = sortMerge($left,$right);
    return $arr;

}
/**
 * [sortMerge 治]
 * @param  array  $left  [description]
 * @param  array  $right [description]
 * @return [type]        [description]
 */
function sortMerge(array $left, array $right):array {
    $tmp = [];
    while(count($left) && count($right)){
        $tmp[] = $left[0] > $right[0]?array_shift($right):array_shift($left);
    }
    echo '<pre>';
    echo 'tmp->';
    print_r($tmp);
    echo 'left->';
    print_r($left);
    echo 'right->';
    print_r($right);
    echo '<hr>';
    echo '</pre>';
    return array_merge($tmp,$left,$right);
}
$R = branch($arr);
echo '<pre>';
print_r($R);
echo '</pre>';