链接列表 - 在当前节点之前插入节点

Question

I'm trying to work on a method that will insert the node passed to it before the current node in a linked list. It has 3 conditions. For this implementation there cannot be any head nodes (only a reference to the first node in the list) and I cannot add any more variables.

1. If the list is empty, then set the passed node as the first node in the list.
2. If the current node is at the front of the list. If so, set the passed node's next to the current node and set the first node as the passed node to move it to the front.
3. If the list is not empty and the current is not at the front, then iterate through the list until a local node is equal to the current node of the list. Then I carry out the same instruction as in 2.

Here is my code.

public class LinkedList 
{
private Node currentNode;
private Node firstNode;
private int nodeCount;

public static void main(String[] args)
 {
 LinkedList test;
 String dataTest;
 test = new LinkedList();
 dataTest = "abcdefghijklmnopqrstuvwxyz";
 for(int i=0; i< dataTest.length(); i++) { test.insert(new String(new char[] { dataTest.charAt(i) }));  }
 System.out.println("[1] "+ test);

  for(int i=0; i< dataTest.length(); i++) { test.deleteCurrentNode(); }
  System.out.println("[2] "+test);

  for(int i=0; i< dataTest.length(); i++)
  {
  test.insertBeforeCurrentNode(new String(new char[] { dataTest.charAt(i) }));
   if(i%2 == 0) { test.first(); } else { test.last(); }
  }

  System.out.println("[3] "+test);
 }

public LinkedList()
{
    setListPtr(null);
    setCurrent(null);
    nodeCount = 0;
}

public boolean atEnd()
{
    checkCurrent();
    return getCurrent().getNext() == null;
}

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

public void first()
{
    setCurrent(getListPtr());
}

public void next()
{
    checkCurrent();
    if (atEnd()) {throw new InvalidPositionInListException("You are at the end of the list. There is no next node. next().");}
    setCurrent(this.currentNode.getNext());
}

public void last()
{
    if (isEmpty()) {throw new ListEmptyException("The list is currently empty! last()");}

    while (!atEnd())
    {
        setCurrent(getCurrent().getNext());
    }

}

public Object getData()
{
    return getCurrent().getData();
}

public void insertBeforeCurrentNode(Object bcNode) //beforeCurrentNode
{
    Node current;
    Node hold;
    boolean done;
    hold = allocateNode();
    hold.setData(bcNode);
    current = getListPtr();
    done = false;
    if (isEmpty())
    {
        setListPtr(hold);
        setCurrent(hold);       
    }

    else if (getCurrent() == getListPtr())
    {
        System.out.println("hi" + hold);
        hold.setNext(getCurrent());
        setListPtr(hold);
    }

    else //if (!isEmpty() && getCurrent() != getListPtr())
    {
        while (!done && current.getNext() != null)
        {
            System.out.println("in else if " + hold);
            if (current.getNext() == getCurrent())
            {
                //previous.setNext(hold);
                //System.out.println("hi"+ "yo" + " " + getListPtr());
                hold.setNext(current.getNext());
                current.setNext(hold);
                done = true; 
            }

            //previous = current;
            current = current.getNext();
        }

    }
    System.out.println(getCurrent());

}

public void insertAfterCurrentNode(Object acNode) //afterCurrentNode
{
    Node hold;
    hold = allocateNode();
    hold.setData(acNode);
    if (isEmpty())
    {
        setListPtr(hold);
        setCurrent(hold);
        //System.out.println(hold + " hi");
    }

    else 
    {
        //System.out.println(hold + " hia");
        hold.setNext(getCurrent().getNext());
        getCurrent().setNext(hold);
    }
}

public void insert(Object iNode)
{
    insertAfterCurrentNode(iNode);
}

public Object deleteCurrentNode()
{
    Object nData;
    Node previous; 
    Node current;
    previous = getListPtr();
    current = getListPtr();
    nData = getCurrent().getData();

    if (isEmpty()) {throw new ListEmptyException("The list is currently empty! last()");}

    else if (previous == getCurrent())
    {
        getListPtr().setNext(getCurrent().getNext());
        setCurrent(getCurrent().getNext());
        nodeCount = nodeCount - 1;
    }

    else 
    {
        while (previous.getNext() != getCurrent())
        {
            previous = current;
            current = current.getNext();


        }
    previous.setNext(getCurrent().getNext());
    setCurrent(getCurrent().getNext());
    nodeCount = nodeCount - 1;  
    }
    return nData;
}

public Object deleteFirstNode(boolean toDelete)
{
    if (toDelete)
    {
        setListPtr(null);
    }
    return getListPtr();
}

public Object deleteFirstNode()
{
    Object deleteFirst;
    deleteFirst = deleteFirstNode(true);
    return deleteFirst;
}

public int size()
{
    return this.nodeCount;
}

public String toString()
{
    String nodeString;
    Node sNode;
    sNode = getCurrent();
    //System.out.println(nodeCount);
    nodeString = ("List contains " + nodeCount + " nodes");
    while (sNode != null)
    {
        nodeString = nodeString + " " +sNode.getData();
        sNode = sNode.getNext();
    }   
    return nodeString;
}

private Node allocateNode()
{
    Node newNode;
    newNode = new Node();
    nodeCount = nodeCount + 1;
    return newNode;
}

private void deAllocateNode(Node dNode)
{
    dNode.setData(null);
}

private Node getListPtr()
{
    return this.firstNode;
}

private void setListPtr(Node pNode)
{
     this.firstNode = pNode;
}

private Node getCurrent()
{
    return this.currentNode;
}

private void setCurrent(Node cNode)
{
    this.currentNode = cNode;
}

private void checkCurrent()
{
    if (getCurrent() == null) {throw new InvalidPositionInListException("Current node is null and is set to an invalid position within the list! checkCurrent()");}
}

/**NODE CLASS ----------------------------------------------*/

    private class Node 
    {
        private Node next; //serves as a reference to the next node 
        private Object data;

        public Node()
        {
            this.next = null;
            this.data = null;
        }


        public Object getData()
        {
            return this.data;
        }

        public void setData(Object obj)
        {
            this.data = obj;
        }

        public Node getNext()
        {
            return this.next;
        }

        public void setNext(Node nextNode)
        {
            this.next = nextNode;
        }

        public String toString()
        {
            String nodeString;
            Node sNode;
            sNode = getCurrent();
            //System.out.println(nodeCount);
            nodeString = ("List contains " + nodeCount + " nodes");
            while (sNode != null)
            {
                nodeString = nodeString + " " +sNode.getData();
                sNode = sNode.getNext();
            }   
            return nodeString;
        }
    }

 }

I have it working for my [1] and [2] conditions. But my [3] (that tests insertBeforeCurrentNode()) isn't working correctly. I've set up print statements, and I've determined that my current is reset somewhere, but I can't figure out where and could use some guidance or a solution.

The output for [1] and [2] is correct. The output for [3] should read

[3] List contains 26 nodes: zxvtrpnljhfdbcegikmoq suwya

Thanks for any help in advance.

Answer 1

In your toString method you start printing nodes from the currentNode till the end of your list. Because you call test.last() just before printing your results, the currentNode will point on the last node of the list, and your toString() will only print 'a'.

In your toString() method, you may want to change

sNode = getCurrent();

with

sNode = getListPtr();

to print your 26 nodes.

Answer 2

For [3] you need to keep pointers to two nodes: one pointer in the "current" node, the one you're looking for, and the other in the "previous" node, the one just before the current. In that way, when you find the node you're looking in the "current" position, then you can connect the new node after the "previous" and before the "current". In pseudocode, and after making sure that the cases [1] and [2] have been covered before:

Node previous = null;
Node current = first;
while (current != null && current.getValue() != searchedValue) {
    previous = current;
    current = current.getNext();
}
previous.setNext(newNode);
newNode.setNext(current);