How to implement merge sorting in php

php method to implement merge sorting: 1. Create a PHP sample file; 2. Define the "public function handle(){...}" method; 3. Through "private function mergeSort($a, $lo, $hi) {...}" method to gradually decompose the data; 4. Use the "merge" method to sort the decomposed data and then merge them together.

The operating environment of this tutorial: Windows 7 system, PHP version 8.1, Dell G3 computer.

php implements the merge sort algorithm

The complexity of the merge sort algorithm is O(nlogn).

The code is as follows, you only need to clone it and execute composer installThen execute php artisan test:mergeSort and you can see the result

    /**
     * 归并排序把数据逐步分解，然后对分解后的数据进行排序，最后合并到一起
     *
     * @return mixed
     */
    public function handle()
    {
        $this->a = [3,70,4,38,5,6,8,4,7,10,6,10,34,4];
        dump($this->a);
        $a = $this->mergeSort($this->a, 0, count($this->a));
        dd($a);
    }
    private function mergeSort($a, $lo, $hi) {
        if (($hi - $lo) < 2) return [$a[$lo]];
        $mi = ($lo + $hi) >> 1;
        //把中点左边的进行归并
        $b = $this->mergeSort($a, $lo, $mi);
        dump(&#39;$b:&#39;,$b);
        //把中点右边的进行归并
        $c = $this->mergeSort($a, $mi, $hi);
        dump(&#39;$c:&#39;,$c);
        //把所有数据进行排序
        return $this->merge($b, $c, $lo,$mi,$hi);
    }
    /**
     * 假设有一个数组$a分成了两个数组[3,4] [2,8]
     * 逐一比较，3and2，取出来2然后3and8取出来3然后4and8取出来4，最后取出来8
     *
     * @param [type] $lo
     * @param [type] $mi
     * @param [type] $hi
     * @return void
     */
    private function merge($b, $c, $lo, $mi, $hi) {
        $lb = $mi - $lo; //$b数组的边界
        $lc = $hi - $mi; //$c数组的边界
        $res = [];
        //$i表示合并后数组的下标 $ib是b数组的下标 $ic是c数组的下标 
        for($i = 0,$ib=0,$ic=0;$ib<$lb || $ic < $lc;){
            //ib 下标没有越界 && c的数组已经空了也就是$ic >= $lc || 比较两个数组首位的大小 如果b的首元素 < c的首元素，那么取出来b的首元素
            if ($ib < $lb && ( $ic >= $lc || $b[$ib] <= $c[$ic])) {
                $res[$i++] = $b[$ib++];
            }
            //k 下标没有越界 && b的数组已经空了也就是$ib >= $lb || 如果c的首元素 < b的首元素，那么取出来c的首元素 
            if ($ic < $lc && ($ib >= $lb || $b[$ib] > $c[$ic])) {
                $res[$i++] = $c[$ic++];
            }
        }
        return $res;
    }

Merge sort Principle

Merge sort is just the opposite of quick sort. It first breaks up the entire array left and right, and then merges and sorts one by one, and finally completes the sorting of the entire array. The sorting diagram is as follows:

First scatter the entire array left and right into a single element, because a single element can be considered ordered.

Corresponding code

if (($hi - $lo) < 2) return [$a[$lo]];
$mi = ($lo + $hi) >> 1;
//把中点左边的进行归并
$b = $this->mergeSort($a, $lo, $mi);
dump(&#39;$b:&#39;,$b);
//把中点右边的进行归并
$c = $this->mergeSort($a, $mi, $hi);
dump(&#39;$c:&#39;,$c);

Next, sort the left and right ordered arrays. Suppose there is an array $a divided into two arrays [3,4] [ 2,8], compare one by one, 3and2, take out 2, then 3and8, take out 3, then 4and8, take out 4, and finally take out 8, the corresponding code:

$lb = $mi - $lo; //$b数组的边界
$lc = $hi - $mi; //$c数组的边界
$res = [];
//$i表示合并后数组的下标 $ib是b数组的下标 $ic是c数组的下标 
for($i = 0,$ib=0,$ic=0;$ib<$lb || $ic < $lc;){
    //ib 下标没有越界 && c的数组已经空了也就是$ic >= $lc || 比较两个数组首位的大小 如果b的首元素 < c的首元素，那么取出来b的首元素
    if ($ib < $lb && ( $ic >= $lc || $b[$ib] <= $c[$ic])) {
        $res[$i++] = $b[$ib++];
    }
    //k 下标没有越界 && b的数组已经空了也就是$ib >= $lb || 如果c的首元素 < b的首元素，那么取出来c的首元素 
    if ($ic < $lc && ($ib >= $lb || $b[$ib] > $c[$ic])) {
        $res[$i++] = $c[$ic++];
    }
}
return $res;

The schematic diagram is as follows:

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