Remove element from linked list java

Question

HI :) I have a program about linked lists and we're supposed to be able to delete two numbers if they are the same.. and i know how to do it from the start but how do you delete two numbers if they are in the middle of the linked list?? All 3 run together Heres my numbers program

import java.util.Scanner;

public class Numbers {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Scanner reader = new Scanner (System.in);
        LinkedList link=new LinkedList();
        LinkedList link2= new LinkedList();
        System.out.println("Enter in 5 numbers to put in your list");
        int num1, num2, num3, num4, num5;
        num1 = reader.nextInt();
        link.addToStart(num1);
        num2 = reader.nextInt();
        link.addToStart(num2);
        num3 = reader.nextInt();
        link.addToStart(num3);
        num4 = reader.nextInt();
        link.addToStart(num4);
        num5 = reader.nextInt();
        link.addToStart(num5);

        link2.addToStart(num5);
        link2.addToStart(num4);
        link2.addToStart(num3);
        link2.addToStart(num2);
        link2.addToStart(num1);

        System.out.println("The size of the linked list is " + link.size());

        System.out.print("Here is the list ");
        link2.outputList();
        System.out.println();
        System.out.print("Here is the list in reverse order ");
        link.outputList( );
        System.out.println();

        if (num1==num2){
             link2.deleteHeadNode(num1);
             link2.deleteHeadNode(num2);
             System.out.println("Here is the list with the removed numbers");
            link2.outputList();
            System.out.println();
            System.out.println("Here is its size");
            System.out.println(link2.size());
        }
        else if (num2==num3){
            link2.deleteHeadNode(num2);
            link2.deleteHeadNode(num3);
            System.out.println("Here is the list with the removed numbers");
           link2.outputList();
           System.out.println();
           System.out.println("Here is its size");
           System.out.println(link2.size());
       }
    }

}

Here is the node program

public class Node1
{
    private Object item;
    private int count;
    private Node1 link;

    public Node1( )
    {
        link = null;
       item = null;
        count = 0;
    }

    public Node1(int num, int newCount, Node1 linkValue)
    {
        setData(num, newCount);
        link = linkValue;
    }

    public void setData(int num, int newCount)
    {
        item = num;
        count = newCount;
    }

    public void setLink(Node1 newLink)
    {
        link = newLink;
    }

    public Object getItem( )
    {
        return  item;
    }

    public int getCount( )
    {
        return count;
    }

    public Node1 getLink( )
    {
        return link;
    }
}

And here is linked lists program

public class LinkedList
{
    private Node1 head;

    public LinkedList( )
    {
        head = null;
    }

    /**
     Adds a node at the start of the list with the specified data.
     The added node will be the first node in the list.
    */
    public void addToStart(int num)
    {
        head = new Node1(num, num, head);
    }

    /**
     Removes the head node and returns true if the list contains at least
     one node. Returns false if the list is empty.
     * @param num1 
    */
    public boolean deleteHeadNode(int num1 )
    {
        if (head != null)
        {
            head = head.getLink( );
            return true;
        }
        else
            return false;
    }

    /**
     Returns the number of nodes in the list.
    */
    public int size( )
    {
        int count = 0;
        Node1 position = head;

        while (position != null)
        {
            count++;
            position = position.getLink( );
        }
        return count;
    }

    public boolean contains(String item)
    {
        return (find(item) != null);
    }

    /**
     Finds the first node containing the target item, and returns a
     reference to that node. If target is not in the list, null is returned.
    */
    private Node1 find(String target)
    {
        Node1 position = head;
        Object itemAtPosition;
        while (position != null)
        {
            itemAtPosition = position.getItem( );
            if (itemAtPosition.equals(target))
                return position;
            position = position.getLink( );
        }
        return null; //target was not found
    }

    public void outputList( )
    {
        Node1 position = head;
        while (position != null)
        {
            System.out.print(position.getItem( ) + " ");
            position = position.getLink( );
        }
    }

    public boolean isEmpty( )
    {
        return (head == null);
    }

    public void clear( )
    {
        head = null;
    }

}

Answer 1

To remove an item in the middle of the linked list, set the previous item's "link" pointer to the "link" pointer of the object you want to remove. For instance, you could add something like this to your LinkedList class:

public void removeNode(Node previousNode, Node nodeToRemove) {
  if (previousNode != null) {
    previousNode.setLink(nodeToRemove.getLink());
  }
}

To think about this better, draw a picture.

 N1 -> N2 -> N3 -> N4

N1's "link" is N2, etc. If you want to remove N2, just set N1's "link" to N3.

 N1 -> N3 -> N4

Answer 2

One approach it to perform a brute force look up.

• For each element you search if is repeated in the list.
• If it is, you remove it
• and go with the next.

As you may see these three steps may be coded quite easy, the point here is to first understand if they do what you want.

This is the pseudo-code for these three points:

forEach( Element a : inList ) do
    // e is the element we want to find repeated.
    forEach( Element b : inList ) do  
         // b is the element in the list.
         if( a == b ) then // repeated
             inList.remove( a ) 
             break;
         endIf
     endFor
 endFor

This approach will allow you to remove all the repeated elements.

Just remember to remove one item, you have to make sure you don't lose the reference it has. So if you have:

n1 -> n2 -> n3

at some point you have to have n1 and n2 pointing to n3 ( that way n1 keeps the reference n2 has )

n1 -> n3  n2 ->n3

and then remove n2 which leaves you:

n1 -> n3

Now how to code that with your specific data structure is a task you have to perform ;)