How to find the first value in a linked list?

问题

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();
   }
}

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;
  }

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!

回答1:

I think you should look at https://docs.oracle.com/javase/7/docs/api/java/util/LinkedList.html and get an idea for the behavior of a linked list initially. Once you have an idea on how it behaves, you can think about how to add functionality around it. Right now you just have a single method which you call more than you should. What also might help is to create an interface and document it so you know what each method should do.

回答2:

You should check that first isn't null in order to do what you describe in the question. Also, it is kind of weird that the first node autoreferences itself because usually it is left in null until you add another node

回答3:

notice that the first value is linked to the first Node with is null. Then you have to check two things

Node == null (you got this)
Node.next == null (you have to do this)

When Node.next == null. It means that Node is first value because it is linked to the initial Node with is null.

Then you have

public static Node getFirst(Node list)
{
    // if the list is empty
    if (list == null)
    {
      System.out.println("Error!");
      System.exit(1);
    } else if(list.link == null) {
      // this is the first value!
      return list;
    } else {
    // keep searching recursive with the next Node
    return getFirst(list.link);
    }
}

回答4:

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.

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.

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

(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");
  }
}

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.

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");
  }

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;
  }

来源：https://stackoverflow.com/questions/53624675/how-to-find-the-first-value-in-a-linked-list

标签

java

methods

linked-list