How to change quicksort to output elements in descending order?
- by masato-san
Hi,
I wrote a quicksort algorithm however, I would like to make a change somewhere so that this quicksort would output elements in descending order.
I searched and found that I can change the comparison operator (<) in partition() to other way around (like below).
//This is snippet from partition() function
while($array[$l] < $pivot) { $l++; }
while($array[$r] > $pivot) { $r--; }
But it is not working..
If I quicksort the array below,
$array = (3,9,5,7);
should be:
$array = (9,7,5,3)
But actual output is:
$array = (3,5,7,9)
Below is my quicksort which trying to output elements in descending order.
How should I make change to sort in descending order? If you need any clarification please let me know. Thanks!
$array = (3,9,5,7);
$app = new QuicksortDescending();
$app->quicksort($array, 0, count($array));
print_r($array);
class QuicksortDescending {
public function partitionDesc(&$array, $left, $right) {
$l = $left;
$r = $right;
$pivot = $array[($right+$left)/2];
while($l <= $r) {
while($array[$l] > $pivot) { $l++; }
while($array[$r] < $pivot) { $r--; }
if($l <= $r) {// if L and R haven't cross
$this->swap($array, $l, $r);
$l ++;
$j --;
}
}
return $l;
}
public function quicksortDesc(&$array, $left, $right) {
$index = $this->partition($array, $left, $right);
if($left < $index-1) { //if there is more than 1 element to sort on right subarray
$this->quicksortDesc($array, $left, $index-1);
}
if($index < $right) { //if there is more than 1 element to sort on right subarray
$this->quicksortDesc($array, $index, $right);
}
}
}
