如何在链表中找到第一个值? [英] How to find the first value in a linked list?

问题描述

我有一个链表，我需要通过getFirst方法在链表中找到第一个值，如果该值为空，我需要显示一条错误消息并退出程序.链接列表已提供给我链接，所以:

I have a linked list I'm given and I need to find the first value in the list via a getFirst method.I need to display an error message and quit the program if the value is null. The linked list is already given to me link so:

class MyLinkedList
{
   private class Node            // inner class
   {
      private Node link;
      private int x;
   }
   //----------------------------------
   private Node first = null;    // initial value is null
   //----------------------------------
   public void addFirst(int d)
   {
      Node newNode = new Node(); // create new node
      newNode.x = d;             // init data field in new node
      newNode.link = first;      // new node points to first node
      first = newNode;           // first now points to new node
   }
   //----------------------------------
   public void traverse()
   {
     Node p = first;
      while (p != null)            // do loop until p goes null
      {
         System.out.println(p.x);  // display data
         p = p.link;               // move p to next node
      }
   }
}
//==============================================
class TestMyLinkedList
{
   public static void main(String[] args)
   {
      MyLinkedList list = new MyLinkedList();
      list.addFirst(1);
      list.addFirst(2);
      list.addFirst(3);
      System.out.println("Numbers on list");
      list.traverse();
   }
}

这是我尝试使用的方法:

Here's what I tried out for the method:

 public static Node getFirst(Node list)
  {
    if (list == null)
    {
      System.out.println("Error!");
      System.exit(1);
    }
    return MyLinkedList.first;
  }

我知道这是不对的，我们只是在我的课堂上开始了，所以我很难理解它是怎么回事.谢谢！

I know this isn't exactly right, we just started this in my class so I'm having trouble understanding what's going on with it. Thank you!

推荐答案

问题中的类MyLinkedList遵循堆栈数据结构的模式(在我编写此答案时).也就是说:每次添加新元素时，新元素都会替换先前添加的元素作为第一个元素.

The class MyLinkedList in your question follows the pattern of a stack data structure(At the time when i am writing this answer). That is: ever time you add a new element, the new element replaces the previously added element as the first element.

我想如果要按顺序添加元素1，2，3，则希望将1作为第一个元素.如果我错了请纠正我.

I guess you want to get 1 as your first element, if you have added elements 1,2,3 in that order. Correct me if i am wrong.

在这种情况下，您的链表及其检索应如下所示:

In that case your linked list and it's retrieval should be like this:

(注意:我避免使用private vars，public getter，settter 等；以使代码易于阅读.但是读者应该添加它们.)

(Note: i have avoided private vars , public getter , settter , etc; to make code easily readable. But readers should add them.)

class Node{ int x; Node next; }

class LinkedList
{ Node head,tail;
  void add(int y)
  { Node node = new Node();
    node.x=y;
    if(head==null)
      head = tail = node;
    else
      tail = tail.next = node;
  }
  int getFirst()
  { if(head!=null)
      return head.x;
    else
      throw new java.util.NoSuchElementException("List is empty");
  }
}

如果您查看java.util.LinkedList，您会发现链表中通常使用的方法.如果这不是家庭作业的问题，那么我建议您不要重新发明轮子.只需使用现有的库即可.

If you look at java.util.LinkedList, you will find methods that are conventionally used in linked lists. If this is not a homework question, then i suggest you do not reinvent the wheel. Just use the existing libraries.

如果必须使用堆栈数据结构，并且无法更改它，那么我建议您必须像这样更改getFirst():

If you have to use the stack data structure, and you cannot change it, then i suggest you have to change your getFirst() like this:

  int getFirst()
  { if(tail!=null)
      return tail.x;
    else
      throw new java.util.NoSuchElementException("List is empty");
  }

如果您不允许在代码中添加Node tail，则您的getFirst()将如下所示:

If you not allowed to add Node tail in your code, then your getFirst() will look like this:

  int getFirst()
  { if(head==null)
      throw new java.util.NoSuchElementException("List is empty");
    Node node = head;
    while(node.next!=null)
      node=node.next;
    return node.x;
  }

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