Question

I am trying to code quicksort in two ways, one in-place, and the other by using separate arrays. I’m kind of stuck on some of the logic, take a look at what I have, Thanks for the help in advance!

public List<Integer> sort(List<Integer> arr){
  if(arr.length > 0)
    List<Integer> ret = new ArrayList<Integer>();
  ret = quickSort(arr);
  return ret;
 }

public List<Integer> quickSort(List<Integer> arr){
  if(arr.length < 2)
    return;

  int pivot = arr[0];
  List<Integer> left = new ArrayList<Integer>();
  List<Integer> right = new ArrayList<Integer>();

  for(int i = 0; i < arr.length; i++){
    if(arr[i] <= pivot)
      left.add(arr[i]);
    else
      right.add(arr[i]);
  }
  quickSort(left);
  quickSort(right);

}

Now i’m stuck, I don’t know what I would do after recursively going through both sets, mostly stuck on how would I connect them together and return a sorted list.

Answer 1

You need to combine left and right sequences together. You need to do it at the end of your algorithm (before the closing }). In pseudo code:

int leftpos = 0, rightpos = 0;
List newlist = new ArrayList();
for(int pos = 0; pos < arr.length; pos++)
  if left[pos] < right[pos] newlist.add(left[leftpos++]);
    else newlist.add(right[rightpos++]);
return newlist;

This is just a pseudo-code. You need to add code to check lengths of each array (left and right) in the for cycle.

Also I must note that this is far from quicksort. So many new array allocations make the algorithm extremely slow and that’s unwelcome when sorting.

Also, right side of line 3 is redundant. You don’t need to allocate anything here, as it is overwritten in the next line. I would just simply replace your lines 3-5 with this:

return quickSort(arr);