Detailed explanation of merge sort in PHP sorting algorithm series_php skills

This article mainly introduces the relevant information of the PHP sorting algorithm series Merge Sort in detail. It has a certain reference value. Interested friends can refer to

Merge Sort

Merge sort (MERGE-SORT) is an effective sorting algorithm based on the merge operation. This algorithm is a very typical application of the divide and conquer method (pide and Conquer). Merge the already ordered subsequences to obtain a completely ordered sequence; that is, first make each subsequence orderly, and then make the subsequence segments orderly. If two ordered lists are merged into one ordered list, it is called a two-way merge.

Merge process

The core of merge sort is how to merge two ordered sequences. Assume that there are two ordered arrays and compare the two ordered arrays. Take the first element, whichever is smaller, and put the element into the third array. After taking it, the element will be deleted in the corresponding array, and so on. When an array is fetched and there are no elements, you can Add the remaining elements of the other array directly to the third array.

Principle

1. Merge every two adjacent numbers in the sequence to form ceil(n/2) sequences. After sorting, each sequence contains two elements, the last sequence may have only one element.

2. Merge the above sequences again to form ceil(n/4) sequences. Each sequence contains four elements, and the last sequence may have only three or less elements.

3. Repeat step 2 until all elements are sorted.

Example

Sort the array [53,89,12,6,98,25,37,92,5]

After the first merge

(53,89),12,(6,98),(25,37),(5,92)

After the second merge

(12,53,89),(6,25,37,98),(5,92)

After the third merger

(6,12,25 ,37,53,89,98),(5,92)

After the fourth merger

5,6,12,25,37,53,89,92,98

PHP code implementation

<?php
function merge_sort($arr){
  $length=count($arr);
  if($length<=1){
    return $arr;
  }
  //分解数组，递归排序
  $half=ceil($length/2);
  $arr2=array_chunk($arr,$half);
  $left=merge_sort($arr2[0]);
  $right=merge_sort($arr2[1]);
  while(count($left)&&count($right)){
    if($left[0]<$right[0]){
      $reg[]=array_shift($left);
    }else{
      $reg[]=array_shift($right);
    }
  }
  return array_merge($reg,$left,$right);
}

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