[我正在用Java构建双向链接列表，当我尝试检索任何信息时，似乎下一个和上一个节点始终为空

Question

我已经调试了好几个小时，但看不到我的搜索方法找不到任何东西的任何原因。而我的toString只会返回第一个节点，然后再返回任何内容。有人能帮我吗？

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，并将始终返回应该是第一个元素的内容，但是否则，我不知道。第一个总是在head.info中出现问题，在调试期间我看到了，但随后的prev和next仍然为空。预先感谢！

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 ) );
  }
}

结果显示为：

7,
Search: null

Answer 1

您的addFirst方法当前未设置新标题的next属性。它必须更像这样：

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

Answer 2

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;
    }
}
}