Quicksort链接列表数据透视表的最后一个元素

Question

I'm trying to implement a quicksort for sorting a CYCLIC LINKED LIST!

There is a DoublyLinkedList which contains ListElements .

The ListElement has the Element First, which has a reference to its predecessor and successor.

eg DoubleLinkyedList in contains the ListElement first. First can be referenced with first.prev oder first.next.

The last Element in the Linked List has a reference to the first element so eg last = first.prev ;

My problem is, I keep getting an error, but I don't know which one precisely, cause it's like a never ending loop and the terminal is not printing the precise exception.

My code of my class Sorter.java where I implement the sorting operation:

package ads1ss13.pa;


public class Sorter {

public DoublyLinkedList quicksort(DoublyLinkedList in, int numOfElements) {


    System.out.println("STARTE sort() in quicksorT()");

    in = Sort(in, in.first.getId(), in.first.prev.getId());

    return in;
}

public DoublyLinkedList Sort(DoublyLinkedList in, int l, int r)
{
    System.out.println("Sort()");

    l = in.first.getId();
    r = in.first.prev.getId();
    ListElement pivot = null;

    System.out.println("l: "+l+"; r: "+r);

    if(l < r)
    {
        pivot = in.first.prev;
    }

    System.out.println("pivot: "+pivot);

    System.out.println("Ausfuehren der Partition mit l, r, pivot");

    int p = Partition(in, l, r, pivot);

    System.out.println("Ausgabe aus Partition p: "+p);

    System.out.println("Ausführen der Sort() mit l und p-1 für r");


    Sort(in, l, p-1);

    System.out.println("Ausführen der Sort() mit p+1 für l, und r");
    Sort(in, p+1, r);

    return in;
}

public int Partition(DoublyLinkedList in, int l, int r, ListElement pivot)
{
    System.out.println("Partition()");

    int i = l;
    int j = r;
    ListElement r_ListElement = in.first.prev;
    ListElement i_element = in.first.next;
    ListElement j_element = pivot;

    System.out.println("i: "+i+"; j: "+j+", pivot: "+j_element);

    do {

        do {
            i = i + 1;
            i_element = i_element.next;
        } while (i_element.getKey() < pivot.getKey());

        do {
            j = j - 1;
            j_element = j_element.prev; 
        } while (j > i || j_element.getKey() > pivot.getKey());

        if(i_element.getKey() < j_element.getKey())
        {
            swap(in, i_element, j_element);
        }

    } while (i < j);

    swap(in, i_element, r_ListElement);

    return i;
}

public void swap(DoublyLinkedList in, ListElement links, ListElement rechts)
{
    if(links.next.getId()==rechts.getId()){

        ListElement Lprev = links.prev;
        ListElement Rnext = rechts.next;
        Lprev.next=rechts;
        rechts.prev=Lprev;
        links.next=Rnext;
        Rnext.prev=links;
        links.prev=rechts;
        rechts.next=links;

        }

        else if(rechts.next.getId()==links.getId())
        {
            swap(in, rechts, links);
        }
        else
        {
            ListElement Lprev=links.prev;
            ListElement Lnext = links.next; 
            links.next=rechts.next;
            links.prev=rechts.prev;
            rechts.next=Lnext;
            rechts.prev=Lprev; 
            links.prev.next=links;
            links.next.prev=links;
            rechts.prev.next=rechts;
            rechts.next.prev=rechts;
        }


        if(in.first.getId()==links.getId())
        {
            in.first = rechts;
        }
        else if(in.first.getId()==rechts.getId())
        {
            in.first = links;

        }
}
}

Do you have any idea why my code never switches to the second recursive call of sort() and which error is returned?

Answer 1

This is hard to analyze because I can't throw the code into a debugger. However, a few things to get you on the right track:

Issue 1.

You take int l and int r as arguments, yet you immediately overwrite them:

l = in.first.getId();
r = in.first.prev.getId();

This is almost certainly not what you want to do, as you will restart over and over again. This is probably what's causing your infinite loop.

Issue 2.

if(l < r)
{
    pivot = in.first.prev;
}

If l >= r , pivot is null , which will cause a NullPointerException

Issue 3.

Your partition logic is wrong. You don't need to have a nested loop. As soon as you compare the element to the pivot, you can immediately do a swap (you will always know which side of the pivot it is supposed to be on).