Doubly Linked List, Insert Before A Given Node in Java

Question

The method not working:

public void insert_before_node(Node givenNode, int data) {
    Node newNode = new Node(data);
    newNode.prev = givenNode.prev;
    givenNode.prev = newNode;
    newNode.next = givenNode;

    if(newNode.prev != null)
        newNode.prev.next = newNode;
}

Another add method which is working:

public void insert_front(int data) {
    Node newNode = new Node(data);
    newNode.next = head;
    newNode.prev = null;

    if(head != null)
        head.prev = newNode;
    head = newNode;
}

A print method to debug:

public void print() {
    Node n = head;
    while(n != null){
        System.out.println(n.data);
        n = n.next;
    }
}

DoublyLinkedList class:

public class DoublyLinkedList {

    static class Node {
        int data;
        Node next;
        Node prev;

        Node(int data) {
            this.data = data;
            this.next = null;
            this.prev = null;
        }
    }

    Node head;

    DoublyLinkedList() {
        this.head = null;
    }

public static void main(String[] args) {

    DoublyLinkedList ll = new DoublyLinkedList();
    ll.insert_front(0);
    ll.insert_before_node(ll.head, 100);

    ll.print();

}
}

LinkedList and Node implementations are very straightforward. Find here: https://www.geeksforgeeks.org/doubly-linked-list/

I first create a linkedlist, insert_front() a value to make the head not null, then use the method above to insert something else. Insertion to front, end, after a node are working, however, this insert_before_node() is not working. What I have inserted with this method is not appears on my print.

I draw on a paper too, still couldn't find the problem.

The geeksforgeeks link also has no java implementation for this method.

Are you sure your insert_front() is working? How does the caller know about the new head of the list? — Robert Kock
– Robert Kock, Commented Oct 9, 2018 at 14:17
The last line of insert_front is useless (head = newNode;) This method should return the new head, otherwise how do you keep track of the new head? You should show us your test code. — StephaneM
– StephaneM, Commented Oct 9, 2018 at 14:17
Sorry, I'm updating the question code. I just wanted to minimize my code. — Melih
– Melih, Commented Oct 9, 2018 at 14:26

rdlopes · Accepted Answer · 2018-10-09 14:43:41Z

Your code is working, apart from the assignment of head in insert_front(Node,int) method, I think you forgot this. before that.

Plus, maybe you would need to

remove the head argument in insert_front method (it's the head of the dll, it has a class member for that),
remove the underscores (not Java good practice, Sonar would complain)
return the nodes you create so you can later reference them (and possibly create a fluent API)

A basic rework would look like this MVP:

import java.util.Objects;

public class DoubleLinkLists {

    public static void main(String[] args) {
        DoubleLinkedList dll = new DoubleLinkedList();

        DoubleLinkedList.Node node5 = dll.insertInFront(5);
        DoubleLinkedList.Node node4 = dll.insertInFront(4);
        DoubleLinkedList.Node node2 = dll.insertInFront(2);
        DoubleLinkedList.Node node1 = dll.insertInFront(1);
        DoubleLinkedList.Node node3 = dll.insertBefore(node4, 3);

        System.out.println(dll);
    }


    public static class DoubleLinkedList {
        Node head;

        @Override
        public String toString() {
            Node current = head;
            StringBuilder sb = new StringBuilder();

            while (current != null) {
                sb.append(current.data)
                  .append(" ");
                current = current.next;
            }

            return sb.toString();
        }

        public Node insertBefore(Node givenNode, int data) {
            Node newNode = new Node(data);
            newNode.prev = givenNode.prev;
            givenNode.prev = newNode;
            newNode.next = givenNode;

            if (newNode.prev != null) {
                newNode.prev.next = newNode;
            }

            return newNode;
        }

        public Node insertInFront(int data) {
            Node newNode = new Node(data);
            newNode.next = head;
            newNode.prev = null;

            if (head != null) {
                head.prev = newNode;
            }

            head = newNode;
            return newNode;
        }

        public static class Node {
            int data;

            Node prev;

            Node next;

            Node(int d) {
                data = d;
            }

            @Override
            public boolean equals(Object o) {
                if (this == o) return true;
                if (o == null || getClass() != o.getClass()) return false;
                Node node = (Node) o;
                return data == node.data;
            }

            @Override
            public int hashCode() {
                return Objects.hash(data);
            }
        }
    }
}

I have updated the code, I just didn't want to share all the code because then nobody reads it(I have already a down-vote and I have no idea why I get a down-vote). However, while trying to trim the code, I made some mistakes. So I added all the code I have used. I was just trying to implement a simple DLL so I didn't care about very clean coding, but I will keep these suggestions in my hand.

Huy Nguyen · Accepted Answer · 2018-10-09 14:24:00Z

1

I edit the code for more readable.

public void insert_before_node(Node next, int data) {
    Node newNode = new Node(data);
    Node prev = next.prev;
    //left to right
    prev.next = newNode;
    newNode.next = next;
    //traverse right to left
    next.prev = newNode;
    newNode.prev = prev;
}

I assume the next and prev is also not null.

By the way, you should add more condition to detect null(next and prev) in insert_before_node. Please update the result and hope it help.

answered Oct 9, 2018 at 14:24

Huy Nguyen

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1 Comment

Melih Over a year ago

It's working, null detection is the problem as I see

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