PHP：快速排序代码一直在说：警告：array_merge（）：参数3不是其中的数组

Question

I am learning about sorting algorithms and I am trying this code:

function quicksort($array){
              if(count($array) == 0){
              return array();
              }

              $pivot = $array[0];
              $left = array();
              $right = array();

                  for($i = 1; $i < count($array); $i++){
                      if($array[$i] < $pivot){
                        $left[] = $array[$i];
                      }
                      else{
                        $right[] = $array[$i];
                      }

                      return array_merge(quicksort($left), array($pivot), quicksort($right));
                  }

              }
              $sorted = quicksort($data);

However, when trying to run it - it gives me:

Warning: array_merge(): Argument #3 is not an array in - on line 21

Any reason for this?

Answer 1

you're returning inside the loop, you need to move that statement outside the loop.

<?php
function quicksort($array){
    if(count($array) == 0){
        return array();
    }

    $pivot = $array[0];
    $left = array();
    $right = array();

    for($i = 1; $i < count($array); $i++){
        if($array[$i] < $pivot){
            $left[] = $array[$i];
        }
        else{
            $right[] = $array[$i];
        }
        // remove return that was here
    }

    // and move it outside the loop here
    return array_merge(quicksort($left), array($pivot), quicksort($right));
}

$data = [3,4,2,1];
print_r(quicksort($data));

 Array ( [0] => 1 [1] => 2 [2] => 3 [3] => 4 )