从单链列表中添加和删除

Question

I thought I had understanding of this in my previous question about linked lists, but I was terribly mistaken, I'm just as lost as I was when I initially posted.

I realize that I'm technically asking two questions, but hopefully getting at least one should make the other easy (assuming they are just reverse of each other).

I have 3 classes already given to me, they are:

SLinkedList.java

package chapter3.linkedList;

    public class SLinkedList<V> {
        // instance variables.  Add the tail reference.
        protected Node<V> head, tail;
        protected long size;

        // methods, empty list constructor first
        public SLinkedList () {
            head = null;
            tail = null;
            size = 0;
        }  // end constructor of a SLinkedList

        // method to add nodes to the list.  Storage space for the node
        // is already allocated in the calling method
        public void addFirst (Node<V> node) {
            // set the tail only if this is the very first node
            if (tail == null)
                tail = node;
            node.setNext (head);    // make next of the new node refer to the head
            head = node;            // give head a new value

            // change our size
            size++;
        }  // end method addFirst

        // addAfter - add new node after current node, checking to see if we are at the tail
        public void addAfter (Node<V>currentNode, Node<V>newNode) {
            if (currentNode == tail)
                tail = newNode;
            newNode.setNext (currentNode.getNext ());
            currentNode.setNext (newNode);

            // change our size
            size++;
        }  // end method addAfter

        // addLast - add new node after the tail node.  Adapted from Code Fragment 3.15, p. 118.
        // Mike Qualls
        public void addLast (Node<V> node) {
            node.setNext (null);
            tail.setNext (node);
            tail = node;
            size++;     
        }  // end method addLast

        // methods to remove nodes from the list.  (Unfortunately, with a single linked list
        // there is no way to remove last.  Need a previous reference to do that.  (See
        // Double Linked Lists and the code below.)
        public Node<V> removeFirst () {
            if (head == null)
                System.err.println("Error:  Attempt to remove from an empty list");

            // save the one to return
            Node<V> temp = head;

            // do reference manipulation
            head = head.getNext ();
            temp.setNext(null);
            size--;

            return temp;

        }  // end method removeFirst

        // remove the node at the end of the list.  tail refers to this node, but
        // since the list is single linked, there is no way to refer to the node
        // before the tail node.  Need to traverse the list.
        public Node<V> removeLast () {
            // // declare local variables/objects
            Node<V> nodeBefore;
            Node<V> nodeToRemove;

            // make sure we have something to remove
            if (size == 0)
                System.err.println("Error:  Attempt to remove fron an empty list");

            // traverse through the list, getting a reference to the node before
            // the trailer.  Since there is no previous reference.
            nodeBefore = getFirst ();

            // potential error  ??  See an analysis and drawing that indicates the number of iterations
            // 9/21/10.  size - 2 to account for the head and tail nodes.  We want to refer to the one before the
            // tail.
            for (int count = 0; count < size - 2; count++)
                nodeBefore = nodeBefore.getNext ();

            // save the last node
            nodeToRemove = tail;

            // now, do the pointer manipulation
            nodeBefore.setNext (null);
            tail = nodeBefore;
            size--;

            return nodeToRemove;

        }  // end method removeLast

        // method remove.  Remove a known node from the list.  No need to search or return a value.  This method
        // makes use of a 'before' reference in order to allow list manipulation.
        public void remove (Node<V> nodeToRemove) {
            // declare local variables/references
            Node<V> nodeBefore, currentNode;

            // make sure we have something to remove
            if (size == 0)
                System.err.println("Error:  Attempt to remove fron an empty list");

            // starting at the beginning check for removal
            currentNode = getFirst ();
            if (currentNode == nodeToRemove)
                removeFirst ();
            currentNode = getLast ();
            if (currentNode == nodeToRemove)
                removeLast ();

            // we've already check two nodes, check the rest
            if (size - 2 > 0) {
                nodeBefore = getFirst ();
                currentNode = getFirst ().getNext ();
                for (int count = 0; count < size - 2; count++) {
                    if (currentNode == nodeToRemove) {
                        // remove current node
                        nodeBefore.setNext (currentNode.getNext ());
                        size--;
                        break;
                    }  // end if node found

                    // change references
                    nodeBefore = currentNode;
                    currentNode = currentNode.getNext ();
                }  // end loop to process elements
            }  // end if size - 2 > 0

        }  // end method remove

        // the gets to return the head and/or tail nodes and size of the list
        public Node<V> getFirst () { return head; }
        public Node<V> getLast () { return tail; }  
        public long getSize () { return size; }

    }  // end class SLinkedList

There's also Node.java

package chapter3.linkedList;

public class Node<V> 
{
    // instance variables
    private V element;
    private Node<V> next;

    // methods, constructor first
    public Node () 
    {
        this (null, null);      // call the constructor with two args
    }  // end no argument constructor
    public Node (V element, Node<V> next) 
    {
        this.element = element;
        this.next = next;
    }  // end constructor with arguments

    // set/get methods
    public V getElement () 
    { 
        return element; 
    }
    public Node<V> getNext () 
    { 
        return next; 
    }
    public void setElement (V element) 
    { 
        this.element = element; 
    }
    public void setNext (Node<V> next) 
    { 
        this.next = next; 
    }

}  // end class Node

and finally GameEntry.java

package Project_1;

public class GameEntry 
{
    protected String name;  // name of the person earning this score
    protected int score;    // the score value
    /** Constructor to create a game entry */
    public GameEntry(String name, int score) 
    {
      this.name = name;
      this.score = score;
    }
    /** Retrieves the name field */
    public String getName() 
    { 
        return name; 
    }
    /** Retrieves the score field */
    public int getScore() 
    { 
        return score; 
    }
    /** Returns a string representation of this entry */
    public String toString() 
    { 
      return name + ", " + score + "\n"; 
    }

}

EDIT POINT I created a driver called Scores.java, in it so far all I have is **I have added what I THINK i need for the classes, I'm probably wrong though:

package Project_1;

import chapter3.linkedList.*;

import java.util.*;


/** Class for storing high scores in an array in non-decreasing order. */
public class Scores 
{

    //add function
    public SLinkedList<GameEntry> add(GameEntry rank, SLinkedList<GameEntry> scores)
    {
        Node<GameEntry> currentNode = scores.getFirst();
        Node<GameEntry> nextNode = null;
        Node<GameEntry> previousNode = null;
        Node<GameEntry> newNode = new Node<GameEntry>();
        newNode.setElement(rank);

        if(scores.getSize() == 0)
        {
            scores.addFirst(newNode);
        }
        else
        {
            while(currentNode != null)
            {               
                nextNode = currentNode.getNext();
                if(nextNode == null)
                {
                    scores.addLast(newNode);
                }
                else
                {
                    scores.addAfter(currentNode, newNode);
                    break;
                }               
            previousNode = currentNode;
            currentNode = currentNode.getNext();
            }
        }
        return scores;
    }

    //remove function
    public void remove(int i)
    {

    }

    //print function
    /*gameenter printing; 
printing=node.Getelement;           //pseudo code for making it work right
print(printing.getscore) 
print(print.getname) 
*/
    public void print(SLinkedList<GameEntry> scores)
    {
        Node<GameEntry> currentNode = scores.getFirst();        
        GameEntry currentEntry = currentNode.getElement();      
        System.out.printf("[");
        for(int i = 0; i < scores.getSize(); i++)
        {
                System.out.printf(", %s", currentEntry.toString());
                currentNode = currentNode.getNext();
                currentEntry = currentNode.getElement();
        }
        System.out.println("]");
    }
}

I have my test driver called ScoresTest.java, that I have pretty much filled out:

package Project_1;

import chapter3.linkedList.SLinkedList;

 public class ScoresTest {
    /**
     * @param args
     */

    public static void main(String[] args) 
    {
        SLinkedList<GameEntry> highScores = new SLinkedList<GameEntry>();  //Linked List for Game Entry
        GameEntry entry;
        Scores rank = new Scores();     
        entry = new GameEntry("Flanders", 681);     
        highScores = rank.add(entry, highScores);
        entry = new GameEntry("Krusty", 324);
        highScores = rank.add(entry, highScores); 
        entry = new GameEntry("Otto", 438);
        highScores = rank.add(entry, highScores); 
        entry = new GameEntry("Bart", 875);
        highScores = rank.add(entry, highScores); 
        entry = new GameEntry("Homer", 12);
        highScores = rank.add(entry, highScores); 
        entry = new GameEntry("Lisa", 506);
        highScores = rank.add(entry, highScores); 
        entry = new GameEntry("Maggie", 980);
        highScores = rank.add(entry, highScores); 
        entry = new GameEntry("Apoo", 648);
        highScores = rank.add(entry, highScores); 
        entry = new GameEntry("Smithers", 150);
        highScores = rank.add(entry, highScores); 
        entry = new GameEntry("Burns", 152);
        highScores = rank.add(entry, highScores); 
        System.out.println("The Original High Scores");
        rank.print(highScores);

        entry = new GameEntry("Moe", 895);
        highScores = rank.add(entry, highScores);
        System.out.println("Scores after adding Moe");
        rank.print(highScores);

        //highScores = rank.remove(4);
        System.out.println("Scores after removing Apoo");
        rank.print(highScores);
    }
}

That's entirely finished, pretty sure I have nothing left to add for it.

I'm not looking for someone to answer it for me, but I have no clue where to start or how to make the add or remove function, in any way. This is an intermediate course, the book does nothing for explaining linked lists (go ahead and look for yourself if you don't believe me, text is called Datastructures and Algorithms in Java, 5th edition). It shows how to do such with an array quite easily...which works perfectly for a linked list, but apparently the teacher does not want us doing it this way, so sadly I am now utterly lost on how to do this.

I've tried looking at other peoples answers on here, and google, and so far nothing has clicked or made any sense at all, I just can't grasp how it works, and the teacher's explanation and example was only to draw boxes on the board, I've never seen a sort, add, or remove function coded for a linked list...can't know what I've not been taught or can't locate.

Any help is greatly appreciated, and thank you in advance!

EDIT

I looked at the import java.util.*; and the commands within it for linked lists, they seem painfully easy. to remove I'd just use list.sublist(i, i).clear(); and the value I wish to remove is removed, super easy, it seems to be just trying to utilize the slinkedlist.java and node.java, I just can't seem to follow them in any way shape or form. I believe the teacher did indeed write them, and I've tried asking for his assistance, stayed 2 hours after class trying to get any understanding from him, and as you can see it did not help much at all. Thank you again for the assistance!

EDIT

I also apologize if this seems like it is vague, but I don't have a specific point where my confusion seems linked, I understand linked lists if we're talking about the java.util.linkedList;, but as far as using what I've been given in this circumstance, I can't follow the logic at all, leaving me quite lost and unsure of where to begin.

Answer 1

In pseudo-code (please note I am not including bound checking etc, simply the logic)

To add a node to the front of the list:

newNode->nextNode = startNode
startNode = newNode

To add to a specific index:

index = 0
currentNode = startNode

// find the node in the list. here you will need to do all kinds of bound checking
while index is less than position
    currentNode = currentNode.nextNode  // move your node pointer to the position
    increment index

// so here we basically insert the new node into the list. what needs to happen is
// to NOT break the list by forgetting the node after the current node. this is why
// we first set the new nodes' next one, to the current nodes' (the one already in
// the list) next node. this way, we still have all the information we need. then,
// when we set the current nodes' next node to the new node, we essentially "break"
// the link and "repair" it by adding the new link.

newNode.nextNode = currentNode.nextNode // some more bound checking required
currentNode.nextNode = newNode

To remove from a specific index:

index = 0
delNode = startNode

// find the node in the list. here you will need to do all kinds of bound checking
while index is less than (position - 1)
    delNode = delNode.nextNode  // move your node pointer to the position
    increment index

delNode.nextNode = delNode.nextNode.nextNode

// that's it. by setting the node's (before the one you whish to delete)
// next node to the node AFTER the one you want to delete, you basically
// "skip" over that node. since it is no longer referenced, the garbage
// collector will take care of the rest. if you wish to return that node
// you can do it quite easily by remembering it.

storeNode = delNode.nextNode                 // some more bound checking required
delNode.nextNode = delNode.nextNode.nextNode // some more bound checking required

// now you still have a reference to the deleted node in storeNode

---

UPDATE

OK, so if I understand correctly, you need to create a linked list that stores scores in a ascending order. As far as I can see, the entire linked list has been implemented for you, you simply need to use the classes provided, and add the logic in Scores.java to keep the list sorted.

First off, I see your nodes are not comparable. If you are at all allowed to change the source given to you, I would suggest having them implement Comparable<Node> and also override the equals(Object o) so that you have logic to compare them. Two nodes can contain the same element, but that does not mean that they are equal.

Please note the change in the method signatures!

//add function
public void add(Node<GameEntry> score) {
    // adding is where you now want to keep everything sorted. so I highly
    // recommend that you implement `Comparable` as I mentioned above. if not,
    // you have to put the logic in here.

    Node<GameEntry> currentNode = highScored.getFirst();
    Node<GameEntry> prevNode = null;

    // if the list is empty, or the new node must go in before the head,
    // simply add it as the head.
    if (highScores.size() == 0 || score.compareTo(currentNode) < 0) {
        highScores.addFirst(score);
    }

    // search for the position of the new node. while the node has a higher score
    // than the current node, we need to continue on so we can place it in the
    // correct place.
    while (currentNode != null && currentNode.compareTo(score) > 0) {
        prevNode = currentNode;
        currentNode = currentNode.getNext();
    }

    // if the currentNode is null, it means it is the highest score, so
    // we can simply add it to the end
    if (currentNode == null) {
        highScores.addLast(score);
    } else {
        // otherwise just add it after the correct node
        highScores.addAfter(prevNode, score);
    }
}


//remove function
public void remove(Node<GameEntry> score) {
    // removing an element should be as described above. if you keep
    // your list sorted during the ADD method, removing any element
    // should not break the order.

    // find the element - removal from a linked list is O(n),
    // since we need to know what the element BEFORE the one
    // is that you want to remove. assuming you have implemented
    // the equals method to check equality of nodes:

    Node<GameEntry> currentNode = highScores.getFirst();
    Node<GameEntry> prevNode = null;
    while (currentNode != null && !currentNode.equals(score)) {
        prevNode = currentNode;
        currentNode = currentNode.getNext();
    }

    // if currentNode is null, the node we wanted to remove was not
    // in the list.
    if (currentNode == null) {
        System.out.println("Node not found");
        return;
    }

    // now, we need to check if there is a node after the one we want
    // to remove.
    if (prevNode.getNext().getNext() != null) {
        // if there is, we follow the logic from the pseudo code
        prevNode.setNext(prev.getNext().getNext());
    } else {
        // if not, we only need to remove the last entry (since the
        // one we want to remove is the last one)
        highScores.removeLast();
    }
}

IMPORTANT

Please just double check the logic here. I did it really quickly without an IDE as I'm not at my development computer at the moment. If anyone finds any issues, please leave a comment and I'll fix it.

If this is not exactly what you asked (your question is a bit vague), let me know.

---

UPDATE 2

Read up on Comparator s here , here and here .