CODEWITHSUNDEEP
javadoubly-linked-list
User
User

Reputation: 1726

How can I add a node at the beginning of a doubly linked list in-between two dummy nodes?

I am trying to write a function addFirst() that takes an item and inserts it at the front of a doubly linked list. The doubly linked list has two dummy nodes, one on each end. The addFirst() method that I wrote so far only returns to two dummy nodes when I iterate through the list and print it. I cannot figure out what is wrong with my code.

public void addFirst(E item) {
        if (item.equals(null)) { throw new NullPointerException(); }

        Node node = new Node(item, null);
        Node ptr = first;

        ptr.next.prev = node;
        node.prev = ptr;
        node.next = ptr.next;

    }

public static void main(String[] args) {

        Deque<Integer> lst = new Deque<Integer>(); // empty list

        lst.addFirst(1);
        lst.addFirst(2);
        lst.addFirst(3);

          Iterator<Integer> it = lst.iterator(); // tests iterator method
            while (it.hasNext()) {
              Integer val = it.next();
              System.out.println(val);
            }    
    }

When I run main, all I get is:

null null

However, I expect:

null 3 2 1 null

Can anybody show me how I can fix my code so that I can add an item at the start of my doubly linked list in between the two dummy first and last nodes?

Upvotes: 0

Views: 684

Answers (1)

User
User

Reputation: 1726

I found what I did wrong. I just needed to complete the chain by adding the line

ptr.next = node;

So my full code for this method looked like this:

public void addFirst(E item) {
if (item.equals(null)) { throw new NullPointerException(); }

        Node node = new Node(item, null);
        Node ptr = first;

        ptr.next.prev = node;
        node.prev = ptr;
        node.next = ptr.next;
        ptr.next = node;

    }

Upvotes: 1

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