CODEWITHSUNDEEP
javaalgorithmsortinglinked-listdoubly-linked-list
jackie
jackie

Reputation: 634

Sort Method for Doubly Linked List

Trying to figure out how to sort my doubly linked list. I get a null pointer exception here:

while (temp.getNext()!=null){

Is there a better approach or any advice to get this going the right way?

public void sort() {
    //bubble sort!
    boolean swapped = (head != null);
    while (swapped) {
        swapped = false;

        EntryNode temp = head;

        //can't swap something with nothing
        while (temp.getNext()!=null){
            if (temp.getLastName().compareTo(temp.getNext().getLastName()) > 0) {
                swapped = true;

                //special case for two nodes
                if (size == 2) {
                    //reassign head and tail
                    tail = temp;
                    head = temp.getNext();
                    tail.setPrev(head);
                    head.setNext(tail);
                    tail.setNext(null);
                    head.setNext(null);
                }
                //if swapping is at head
                else {

                    if (temp == head) {
                        head = temp.getNext();
                        temp.setNext(head.getNext());
                        head.getNext().setPrev(temp);
                        head.setPrev(null);
                        head.setNext(temp);
                        temp.setPrev(head);
                    }

                    else {
                        temp.setNext(temp.getNext().getNext());
                        temp.setPrev(temp.getNext());
                        temp.getNext().setNext(temp);
                        temp.getNext().setPrev(temp.getPrev());
                    }
                }
            }
            //loop through list
            temp = temp.getNext();
        }
    }
}

Upvotes: 3

Views: 6057

Answers (3)

Óscar López
Óscar López

Reputation: 236124

Use the merge sort algorithm, is often the best choice for sorting a (single or doubly) linked list. There's already a post discussing the relevant implementation issues.

Upvotes: 2

Stephen C
Stephen C

Reputation: 719336

The simple approach is to put the list contents into an array, use Arrays.sort to sort the array, and finally rebuild the list from the sorted array.

Upvotes: 0

torrential coding
torrential coding

Reputation: 1765

I think you should check for:

while(temp != null)

because you are already assigning

temp = temp.getNext()

at the end of the while loop.

Upvotes: 1

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