Java 双链表前一个指针未正确分配

Question

我正在尝试用 Java 定义我自己的双向链表类。我当前的方法将整数添加到 LinkedList (按从最低值到最高值的顺序)，从最低到最高打印它们，然后从最高到最低打印它们。我从最高到最低打印的方法必须通过使用当前节点指向前一个节点的指针来知道接下来要打印哪个数字。但是，由于某种原因，我的 add 方法没有正确分配我以前的节点指针。 LinkedList 类看起来像

public class DoubleLink {
    /**
     * Class to create a node object for double linked list
     * @author Nick Gilbert
     */
    private class Node {
        int num;
        Node next;
        Node previous;

        public Node(int num, Node next, Node previous) {
            this.num = num;
            this.next = next;
            this.previous = previous;
        }
    }
    private Node head;

    public DoubleLink() { //All lists start out empty
        head = null;
    }

    public void add(int num) {
        if(head == null) //If num is first entry to list
        {
            head = new Node(num, null, null);
            return;
        }

        Node current = null; //Pointer to iterate through list
        for(current = head; current.next != null; current = current.next){ //Starting at head and going to end
            if(current.next.num >= num) { //if next number is greater than number we want to add then we've found the spot to add it to
                current.next = new Node(num, current.next, current);

                if(num < head.num) //If this statement is true then the num we added is less than the current head's num which means the head needs to be reassigned
                {
                    int temp = current.next.num;
                    current.next.num = head.num;
                    head.num = temp;
                }
                return;
            }
        }
        current.next = new Node(num, null, current);
    }

    public void print() { //prints from lowest number to highest
        for(Node current = head; current != null; current = current.next) {
            System.out.println(current.num );
        }
    }

    public void printBackwards() { //prints from highest to lowest
        Node current = null;
        for(current = head; current != null; current = current.next) {
            if(current.next == null)
                break;
        }
        Node tail = new Node(current.num, null, current.previous);
        for(current = tail; current != null; current = current.previous) {
            System.out.println(current.num );
        }
    }
}

下一个代码段包含我的测试用例

public class DoubleLinkTester {

    public static void main(String[] args) {
        DoubleLink list = new DoubleLink();
        list.add(5);
        list.add(9);
        list.add(7);
        list.add(3);
        list.add(8);
        list.add(10);
        list.add(4);
        list.add(6);
        System.out.println("Call print, should get 3,4,5,6,7,8,9,10");
        list.print();
        System.out.println("Call printBackwards, should get 10,9,8,7,6,5,4,3");
        list.printBackwards();

使用上述测试用例执行 printBackwards 方法会打印 10, 9, 3，而不是预期的 10, 9, 8, 7, 6, 5, 4, 3。我相信这是因为 add 方法没有正确重新分配添加新数字时指向以前数字的指针，但我不知道如何修复它。

Answer 1

您错过了将下一个节点的上一个节点重置为添加的新节点。

public void add(int num) {
    // Code above this remains same

            if(current.next.num >= num) { //if next number is greater than number we want to add then we've found the spot to add it to

                Node actualNext = current.next;    // Missed

                current.next = new Node(num, current.next, current);

                actualNext.previous = current.next;   //Missed

 // Code below this remains same
}