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------to solve the idea----------------------
Each execution is a local variable in the function body, unless it is a static or global variable that changes
------to solve the idea----------------------
This is a clear view.

print_r (Quick_sort (Array (6,1,5,8,7,4,2,3,0))); 
 Function Quick_sort ($arr) {
//Determine whether you need to proceed with 
 $length = count ($arr) first; 
 if ($length <= 1) {
 return $arr; 
} 
//If not returned, indicates that there are more than 1 elements in the array, you need to sort 
//Select a ruler 
//Select the first element 
 $base _num = $arr [0]; 
 Iterate through all elements except the ruler, place the two array in the size relationship 
//Initialize two arrays 
 $left _array = Array (),//less than the ruler's 
 $right _array = Array (),////greater than the ruler's 
 for ($i =1; $i < $length; $i + +) {
 if ($base _num > $arr [$i]) {
//put left array 
 $left _array[] = $arr [$i]; 
} else {
//Put right array 
 $right _array[] = $arr [$i]; 
} 
} 
 printf ("in[%s] b[%d] l[%s] r[%s] 
", Join (', ', $arr), $base _num, join (', ', $left _array), join (', ', $ Right_array)); 
//The same sorting is done for the left and right groups 
//recursively call this function and record the result 
 $left _array = Quick_sort ($left _array); 
 $right _array = Quick_sort ($right _array); 
//merge left, ruler, right 
 return Array_merge ($left _array, Array ($base _num), $right _array); 
} 

in[6,1,5,8,7,4,2,3,0] b[6] l[1,5,4,2,3,0] r[8,7]
in[1,5,4,2,3,0] b[1] l[0] r[5,4,2,3]
in[5,4,2,3] b[5] l[4,2,3] r[]
in[4,2,3] b[4] l[2,3] r[]
in[2,3] b[2] l[] r[3]
in[8,7] b[8] l[7] r[]

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