PHP sorting algorithm: PHP quick sort algorithm principle and algorithm implementation

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php quick sorting algorithm principle: Find any element in the current array (generally select the first element), as a standard, create two empty arrays left and rignt, and traverse the entire array elements. If the traversed Elements that are smaller than the current element are placed in the array left, elements that are larger than the current element are placed in rigt, and then the same operation is performed on the new array.

Recursion:
Recursion is a mechanism by which a function calls itself.
Recursion must have boundary conditions, which is the recursive exit (exit the recursion)
The recursive forward section and the recursive return section, which are the final values ​​
When the boundary conditions are not met, the recursion advances; when the boundary If the condition (recursive exit) is met, recursion returns.
PHP's recursion consumes very much performance, so try to avoid using it.

php quick sorting principle compound recursion principle
Recursion point: If the array element is greater than 1, it needs to be decomposed, so our recursion point is that the number of newly constructed array elements is greater than 1
Recursive exit: When the number of array elements is 1, there is no need to sort the new array.

php quick sort method implementation code:

$arr = [34,56,7,89,12,9];
function quick_sort($arr)
{
// 判断参数是否是一个数组
if(!is_array($arr)) return false;
// 递归出口：数组长度为1，直接返回数组
$length = count($arr);
if($length <= 1) return $arr;
// 数组元素有多个，则定义两个数组
$left = $right = [];
// 循环遍历数组，把第一个元素当做比较的对象
for($i=1;$i<$length;$i++)
{
    //判断当前元素的大小
    if($arr[$i] < $arr[0])
    {
        $left[] = $arr[$i];
    }
    else
    {
        $right[] = $arr[$i];
    }
}
// 递归调用
$left = quick_sort($left);
$right = quick_sort($right);
// 将所有的结果合并
return array_merge($left,[$arr[0]],$right);
}
print_r(quick_sort($arr));

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