如何在第一个元素之后实现双向链表

Question

I'm trying to create a double linked list in Java. With the following elements inside each node:

public class Node {
    int data;
    Node next;
    Node before;
}

data is just a number (0 to 9). Node next indicates the next pointer in a list, and Node before the before pointer in a list.

In the first loop where i ask for list == null is the first element in a list, which i indicate that the new.before = null so its the first element in the linked list. And the new.next=list so it can continue building the list without entering the loop again.

But i'm stuck when i try to link the other elements in the list, not the first one . I have the idea which i need to link the element before and the next element. But i dont know how to implement this idea.

This is the code for generating the doubly linked list.

public static Node CreateDoubleLinkedList(Node list, int data)
    {
        Node nuevo=new Node();
        new.data=data;
        new.next=null;
        new.before=null;
        Node aux=list;
        
        if(list==null)
        {
            list=new;
            new.next=list;
            new.before= null;
            return list;
        }
        else
        {
            /* I dont know how to implement the idea of the doubly linked list after the first element*/
            new.before=aux;
            while(aux.next!=null)
            {
                aux=new.next;
            }
            return list;
        }   
    }

Answer 1

Okay, so when attaching nodes to a doubly-linked list there are more links, and therefore more cases to be concerned about.

(1) The list is empty. Add the incoming node to the head of the list. The next pointer and the before pointer point to nothing (null)

(2) There is one node in the list, and we append the incoming node to the front of the list. Here we append the incoming node's next to the head of the list, and the node that was already there points the before pointer back at the incoming node.

(3) Okay, now assume the incoming node goes in between two nodes. This is more complicated. Attach the incoming node's before to the previous node, and then attach the incoming node's next to the node that's next in the list. Now, relink the previous node's next to point at the incoming node, and then do the same with the node in front, but this time with the before link.

(4) Okay, one last case to be concerned about. Append the incoming node at the end of the list. This is similar to appending to the front of the list, but now, we use the incoming node's back pointer to point at the previous node, and like all the other cases, don't forget to update the link at the previous node. In other words, point the previous node's next pointer at the incoming node.