如何创建双向链表添加方法？

Question

它是一个 Double 类型的双向链表。 我的其他方法不会与测试仪一起运行，因为他们说“this.head”或“this.current”是 null 所以我假设 add 方法有问题，因为它首先被使用。 它正在抛出任何帮助表示赞赏！

{
    private class ListNode
    {
        double data;
        ListNode prev;
        ListNode next;
    
        public ListNode(Double aData, ListNode foreWards, ListNode backWards)
        {
            data = aData;
            next = foreWards;
            prev = backWards;
        }
    }
    private ListNode head;
    private ListNode current;
    private ListNode previous;
    private ListNode tail;
    public DoubleDoubleLL()
    {
        head = current = previous = null;
    }
public void add(Double aData)
    {
        ListNode newNode = new ListNode(aData, null, null);
        if(head == null)
        {
            head = current = tail = newNode;
            tail = current = newNode;
            return;
        }
        else
        {
            tail.next = newNode;
            newNode.prev = tail;
            tail = newNode;
            tail.next = null;
            ListNode temp = head;
            temp = temp.next;
            temp.next = newNode;
            temp.prev = newNode;
        }
    }
}

Answer 1

首先，在第一条和最后一条记录的情况下，next 或 prev == null 是正常的。 这就是我们指示列表开始和结束的方式。

至于你的添加方法，是的，它是错误的:)

请参阅此处的示例，我添加了一个带有一些打印输出的主要方法，以便您了解它是如何工作的。

public class DoubleLinkedList {
    private ListNode head;
    private ListNode tail;
    private int size = 0;

    private class ListNode {
        double data;
        ListNode prev;
        ListNode next;

        public ListNode(double data, ListNode prev, ListNode next) {
            this.data = data;
            this.prev = prev;
            this.next = next;

        }
        public String toString(ListNode node) {
            return node == null ? "NIL" : ""+node.hashCode();
        }
        public String toString() {
            return String.format("ListNode [%d] [data: %f, next: %s, prev: %s]", this.hashCode(), this.data, this.toString(this.next),this.toString(this.prev));
        }
    }

    public DoubleLinkedList() {
    }

    public void add(double data) {
        this.size++;
        ListNode node = new ListNode(data, this.tail, null);
        if (this.tail != null) {
            this.tail.next = node;
        }
        this.tail = node;
        if (this.head == null) {
            this.head = node;
        }
    }


    public String toString(ListNode node) {
        return node == null ? "NIL" : ""+node.hashCode();
    }
    public String toString() {
        return String.format("DoubleLinkedList [%d] [size: %d, head: %s, tail: %s] (%s)", this.hashCode(), this.size, toString(this.head), toString(this.tail), this.printList());
    }

    private String printList() {
        StringBuilder sb = new StringBuilder();
        ListNode next = this.head;
        while (next != null) {
            sb.append("\n\t").append(next.toString()).append(",");
            next = next.next;
        }
        return sb.toString();
    }

    public static void main(String[] args) {
        DoubleLinkedList list = new DoubleLinkedList();
        System.out.println(list.toString());

        list.add(11);
        System.out.println(list.toString());
        list.add(12);
        System.out.println(list.toString());
        // adding two at once
        list.add(13);
        list.add(14);
        System.out.println(list.toString());
    }
}