我正在用Java构建一个Double Linked List，当我试图检索任何信息时，下一个和上一个节点似乎总是空的

Question

我已经调试了几个小时了，我看不出为什么我的搜索方法找不到任何东西。我的toString只返回第一个节点，然后什么都不返回。有人能帮我吗？

在调试时，我可以确认列表的顺序是正确的，我可以在addLast和addFirst之间切换，并且总是返回应该是第一个元素的内容，但除此之外，我不知道。第一个总是在head.info中出现，在调试过程中我看到了这一点，但prev和next仍然为空。提前感谢！

public class DoubleLinkedList {
private DoubleNode head;
public DoubleLinkedList() {
head = null;
}
public class DoubleNode {
int info;
DoubleNode prev;
DoubleNode next;
public DoubleNode(int key) {
info = key;
prev = next = null;
}
}

public DoubleNode search(int key) {
DoubleNode current = this.head;
while (current != null && current.info != key) {
current = current.next;
}
return current;
}
public void addFirst(int key) {
this.head = new DoubleNode(key);
}
public void addLast(int key) {
DoubleNode node = new DoubleNode(key);
DoubleNode current;
if (head == null) {
this.head = node;
} else {
current = this.head;
while (current.next != null) {
current = current.next;
current.next = node;
node.prev = current;
}
}
}
public int delete(int key) {
DoubleNode current, sent;
current = search( key );
if (current != null) {
sent = delete( current );
return sent.info;
} else {
return -1;
}
}
private DoubleNode delete(DoubleNode node) {
if (node.prev != null) {
(node.prev).next = node.next;
} else {
this.head = node.next;
}
if (node.next != null) {
(node.next).prev = node.prev;
}
return node;
}
public String toString() {
String string = "";
while (head != null) {
string += head.info + " ";
head = head.next;
}
return string;
}
public static void main(String[] args) {
DoubleLinkedList test = new DoubleLinkedList();
test.addLast( 3 );
test.addLast( 5 );
test.addFirst( 7 );
System.out.println(test);
System.out.println( "Search: " + test.search( 1 ) );
}
}

结果如下：

7,
Search: null

Answer 1

您的addFirst方法当前没有设置新头的next属性。它需要更像这样：

public void addFirst(int key) {
DoubleNode node = new DoubleNode(key);
node.next = this.head;
this.head = node; 
}

Answer 2

看看这个例子：

public class Test {
public static void main(String[] args) {
List list = new List();
list.addNode("1");
list.addNode("2");
list.addNode("3");
System.out.println(list);// not implemented
}
}
class List {
Node head;
Node tail;
class Node {
Node next;
Node previous;
String info;
public Node(String info) {
this.info = info;
}
}
void addNode(String info) {
Node node = new Node(info);
if (head == null) {
head = tail = node;
} else if(tail == head){
Node next = new Node(info);
tail.next = next;
head = next;
head.previous = tail;
} else{
Node next = new Node(info);
Node current = head;
head.next =next;
head = next;
head.previous = current;
}
}
}