双链接列表不会向后打印

Question

在下面的双向链表示例中，我可以将节点添加到双向链表的前面和双向链表的末尾。我还可以向前遍历双向链表并成功打印节点的值。当我向后打印列表时，我的tail.previous值为空，我只能打印当前在尾部的节点值。请告诉我出了什么问题。谢谢。

public class DLL {

public Node head;
public Node tail;

/* Doubly Linked list Node*/
public class Node { 
    public int data; 
    public Node prev; 
    public Node next; 

    // Constructor to create a new node 
    // next and prev is by default initialized as null 
    Node(int d) { data = d; } 
} 

public void addToFront(int data){
    System.out.println( data + " will be added to the front!");
    Node nn = new Node(data);
    nn.next = head;
    nn.prev = null;
    if (head != null) head.prev = nn;
    head = nn;
    if (tail == null) tail = nn;
}

public void addToBack(int data){
    Node nn = new Node(data);
    System.out.println(data + " will be added to the back!");
    if (tail != null) tail.next = nn;
    tail = nn;
    if (head == null) head = nn;
}

public void printForward(){

    System.out.println("Printing forward!");
    Node runner = head;
    while(runner != null){
        System.out.println(runner.data);
        runner = runner.next;
    }

}
public void printBackward(){

    System.out.println("Printing backwards");
    Node runner = tail;
    while (runner != null){
        System.out.println(runner.data);
        runner = runner.prev;      
    }

 }
}

测试代码如下：公共类DDLTest{

public static void main (String[] args){
    DLL dl = new DLL();
    dl.addToFront(2);
    dl.addToFront(1);
    dl.addToBack(3);
    dl.printForward();
    dl.printBackward();
 }
}

Answer 1

你必须用addToback方法指出

public void addToBack(int data){
    Node nn = new Node(data);
    System.out.println(data + " will be added to the back!");
    if (tail != null){
        tail.next = nn;
        nn.prev = tail;
        tail = nn;
    }


    if (head == null) head = nn;
}

Answer 2

addToback方法没有设置新节点的prev指针。

添加以下内容：

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