Hey ya'll I am having a little trouble with my singly linked list. I decided to create a simple one because we do not get enough practice during my data structures class and cannot seem to find why I am not getting the right output.
The code is:
package linked_list;
public class LinkedList {
private Node head;
private Node tail; // After figuring out head, come back to this FIXME
private int listSize;
public LinkedList() {
head = new Node(null);
tail = new Node(null);
}
public void addLast(String s) {
Node newNode = new Node(s);
if (head == null) {
addFirst(s);
} else {
while (head.next != null) {
head = head.next;
}
head.next = newNode;
tail = newNode;
}
listSize++;
}
public void addFirst(String s) {
Node newNode = new Node(s);
if (head == null) {
head = newNode;
tail = newNode;
}
else {
newNode.next = head;
head = newNode;
}
listSize++;
}
public Object getFirst() {
return head.data;
}
public Object getLast() {
return tail.data;
}
public void clear() {
head = null;
tail = null;
listSize = 0;
}
public Object peek() {
try {
if (head == null) {
throw new Exception ("The value is null");
}
else {
return head;
}
} catch (Exception e) {
System.out.println(e.getMessage());
return null;
}
}
public int size() {
return listSize;
}
// This class has the ability to create the nodes that are used
// in the Linked List.
private class Node {
Node next;
Object data;
public Node(String value) {
next = null;
data = value;
}
public Node(Object value, Node nextValue) {
next = nextValue;
data = value;
}
public Object getData() {
return data;
}
public void setData(Object dataValue) {
data = dataValue;
}
public Node getNext() {
return next;
}
public void setNext(Node nextValue) {
next = nextValue;
}
}
}
Now here is my driver that I created to run a simple little operation:
package linked_list;
public class LinkedListDriver {
public static void main(String[] args) {
LinkedList list1 = new LinkedList();
list1.clear();
list1.addLast("This goes last");
list1.addFirst("This goes first");
list1.addLast("Now this one goes last");
System.out.println(list1.getFirst());
System.out.println(list1.getLast());
}
}
My output is this:
This goes last
Now this one goes last
I guess my question is why am I not getting the answer This goes first from my getFirst() method. It seems to be something wrong with the order or structure of that method but I cannot pinpoint it.
When you are in the else in the addLast, you are changing the reference to head. You should use another reference pointer to traverse the list when adding in the else.
Also, your list size should only be incremented in the else in addLast because you are incrementing twice otherwise (once in addFirst and again after the if-else in addLast).
Related
So I wrote my own linked list (and list node) in Java as a part of a homework.
Now, I'm trying to erase entries, but the function is not working.
I know the concept:
Search for node keeping the previous;
Tell previous node to point to next node;
Return or stop using the node so GC erases it.
For some reason it is not working. I can delete the node with the same value over and over. I'm afraid it is something related to Java pointers.
The code:
Node:
public class SimpleNode<E> {
private E value;
private SimpleNode<E> next;
public SimpleNode() {
this.value = null;
this.next = null;
}
public NoSimples(E data, SimpleNode<E> ref) {
this.value = data;
this.next = ref;
}
// Getters and Setters
}
List:
public class LinkedList<E> implements Iterable<SimpleNode<E>> {
private SimpleNode<E> head;
private int size = 0;
public LinkedList() {
this.head = new SimpleNode<E>();
}
public void add(SimpleNode<E> node) {
this.addFirst(node.getValue());
}
public void addFirst(E item) {
SimpleNode<E> nonde = new SimpleNode<E>(item, this.head);
this.head = node;
size++;
}
public void add(E value) {
this.addFirst(value);
}
public SimpleNode<E> removeFirst() {
SimpleNode<E> node = this.head;
if (node == null) {
return null;
} else {
this.head = node.getNext();
node.setNext(null);
this.size--;
return node;
}
}
public SimpleNodes<E> remove(E value) {
SimpleNode<E> nodeAnt = this.head;
SimpleNode<E> node = this.head.getNext();
while (node != null) {
if (node.getValue()!= null && node.getValue().equals(value)) {
nodeAnt.setNext(node.getNext());
node.setNext(null);
return node;
}
nodeAnt = node;
node = node.getNext();
}
return null;
}
// Other irrelevant methods.
}
Multiple Problems :
Think if you have a list 1,2,3,4. Now, if you try to remove 1, your code fails.
nodeAnt = node should be nodeAnt = nodeAnt.getNext(). Remember, the're all references, not Objects
Also, a recursive way might be easier to understand. For example, Here is how I implemented it
public void remove(E e){
prev = head;
removeElement(e, head);
System.gc();
}
private void removeElement(E e, Node currentElement) {
if(currentElement==null){
return;
}
if(head.getData().equals(e)){
head = head.getNext();
size--;
}else if(currentElement.getData().equals(e)){
prev.setNext(currentElement.getNext());
size--;
}
prev = prev.getNext();
removeElement(e, currentElement.getNext());
}
Note: I delete all occurrences of the Element, as I needed it. You may need it to be different.
I already wrote a small program of single linked list with add and traverse method in that. Now I want to convert it into a doubly linked list. I know all the concept of doubly linked list but I am facing little difficulty to implement it in my program.
public class SingleLinkList<T> {
private Node<T> head;
private Node<T> tail;
public void add(T element)
{
Node<T> nd = new Node<T>();
nd.setValue(element);
if (head==null)
{
head = nd;
tail = nd;
}
else
{
tail.setNextRef(nd);
tail = nd;
}
}
public void traverse(){
Node<T> tmp = head;
while(true){
if(tmp == null){
break;
}
System.out.println(tmp.getValue());
tmp = tmp.getNextRef();
}
}
public static void main (String args[])
{
SingleLinkList<Integer> s1 = new SingleLinkList<Integer>();
s1.add(2);
s1.add(3);
s1.add(3);
s1.traverse();
}
}
class Node<T> {
private T value;
private Node<T> nextRef;
public T getValue() {
return value;
}
public void setValue(T value) {
this.value = value;
}
public Node<T> getNextRef() {
return nextRef;
}
public void setNextRef(Node<T> nextRef) {
this.nextRef = nextRef;
}
public int compareTo(T arg)
{
if (arg==this.value)
{
return 0;}
else
{return 1;}
}
}
Add a Node<T> prevRef field to your list class with appropriate getters and setters and then add this method:
public void linkReverse(Node<T> head) {
if (head == null) {
return;
}
head.setPrevRef(null);
if (head.getNextRef() == null) {
return;
}
Node<T> prev = head;
Node<T> curr = head.getNextRef();
while (curr != null) {
curr.setPrevRef(prev);
prev = curr;
curr = curr.getNextRef();
}
}
This method will walk down a currently singly linked list and will link each node in reverse, leaving the list doubly linked.
Of course, you would need to modify the other methods as well, but this is at least a good starting point.
just add private Node<T> prevRef; instance variable to Node class, and set it during add() method. I suggest that traverse() will receive a boolean (or even better, enum) direction argument
public class LinkedList<T>
{
private Node head;
private int size;
public LinkedList()
{
}
public void addToHead(T value) // create new node, make new node point to head, and head point to new node
{
if (head == null)
{
head = new Node(value,null);
}
else
{
Node newNode = new Node(value,head);
head = newNode;
}
size++;
}
public boolean isEmpty()
{
return head == null;
}
public int size()
{
return size;
}
public void removeHead()
{
head = head.next;
size--;
}
public void addToTail(T value)
{
if (isEmpty())
{
System.out.println("You cannot addtoTail of a emptyList!");
}
else
{
System.out.println(value);
Node current = head;
System.out.println("we are pointing to head: "+current);
while (current.getNext() != null) // loop till the end of the list (find the last node)
{
System.out.println("we are now pointing to: "+current.getElement());
current = current.getNext();
}
System.out.println("We are at the last node:"+current); // its working
System.out.println("it should point to null:"+current.getNext()); // its working
current.setNext(new Node(value,null)); // make it point to our new node we want to insert
System.out.println(current.getNext()); // it is pointing to the new node.. yet the node is not actually inserted (local variable problem? )
size++;
}
}
public String toString()
{
String output = "";
if (!isEmpty())
{
Node current = head;
output = "";
while (current.getNext() != null)
{
output += current.toString()+ "->";
current = current.getNext();
}
}
return output;
}
protected class Node
{
private T element;
private Node next;
public Node()
{
this(null,null);
}
public Node(T value, Node n)
{
element = value;
next = n;
}
public T getElement()
{
return element;
}
public Node getNext()
{
return next;
}
public void setElement(T newElement)
{
element = newElement;
}
public void setNext(Node newNext)
{
next = newNext;
}
public String toString()
{
return ""+element;
}
}
}
So I have written this linkedlist class, and every method works except addtoTail. For example say I create a instance of my linkedlist class, and call addToHead(5), then addtoTail(6) and use my toString method to print out the linkedlist, it only contains 5->. I debugged the addToTail and everything seems to be pointing to the correct locations, yet for some reason it does not add the new node (6) to the list. Hopefully I explained that clearly. I am probably missing something really simple (I even drew it on paper to visualize it but do not see the problem).
Your addToTail function is probably fine. I think the culprit is your toString function. In particular, in this snippet:
while (current.getNext() != null)
{
output += current.toString()+ "->";
current = current.getNext();
}
Your condition terminates the loop before reaching the end. What you actually want is:
while(current != null) {
....
}
I have been diligently watching YouTube videos in an effort to understand linked lists before my fall classes start and I am uncertain how to proceed with iterating over the following linked list. The 'node' class is from a series of videos (same author), but the 'main' method was written by me. Am I approaching the design of a linked list in an illogical fashion (assuming of course one does not wish to use the predefined LinkedList class since the professor will expect each of us to write our own implementation)?:
class Node
{
private String data;
private Node next;
public Node(String data, Node next)
{
this.data = data;
this.next = next;
}
public String getData()
{
return data;
}
public Node getNext()
{
return next;
}
public void setData(String d)
{
data = d;
}
public void setNext(Node n)
{
next = n;
}
public static String getThird(Node list)
{
return list.getNext().getNext().getData();
}
public static void insertSecond(Node list, String s)
{
Node temp = new Node(s, list.getNext());
list.setNext(temp);
}
public static int size(Node list)
{
int count = 0;
while (list != null)
{
count++;
list = list.getNext();
}
return count;
}
}
public class LL2
{
public static void main(String[] args)
{
Node n4 = new Node("Tom", null);
Node n3 = new Node("Caitlin", n4);
Node n2 = new Node("Bob", n3);
Node n1 = new Node("Janet", n2);
}
}
Thanks for the help,
Caitlin
There are some flaws in your linked list as stated by some of the other comments. But you got a good start there that grasps the idea of a linked list and looks functional. To answer your base question of how to loop over this particular implemention of the linked list you do this
Node currentNode = n1; // start at your first node
while(currentNode != null) {
// do logic, for now lets print the value of the node
System.out.println(currentNode.getData());
// proceed to get the next node in the chain and continue on our loop
currentNode = currentNode.getNext();
}
Maybe this will be useful:
static void iterate(Node head) {
Node current = head;
while (current != null) {
System.out.println(current.getData());
current = current.getNext();
}
}
// or through recursion
static void iterateRecursive(Node head) {
if (head != null) {
System.out.println(head.getData());
iterateRecursive(head.getNext());
}
}
class List {
Item head;
class Item {
String value; Item next;
Item ( String s ) { value = s; next = head; head = this; }
}
void print () {
for( Item cursor = head; cursor != null; cursor = cursor.next )
System.out.println ( cursor.value );
}
List () {
Item one = new Item ( "one" );
Item two = new Item ( "three" );
Item three = new Item ( "Two" );
Item four = new Item ( "four" );
}
}
public class HomeWork {
public static void main( String[] none ) { new List().print(); }
}
Good luck!!
You can have your linked list DS class implement 'Iterable' interface and override hasNext(), next() methods or create an inner class to do it for you. Take a look at below implementation:
public class SinglyLinkedList<T>{
private Node<T> head;
public SinglyLinkedList(){
head = null;
}
public void addFirst(T item){
head = new Node<T>(item, head);
}
public void addLast(T item){
if(head == null){
addFirst(item);
}
else{
Node<T> temp = head;
while(temp.next != null){
temp = temp.next;
}
temp.next = new Node<T>(item, null);
}
}
private static class Node<T>{
private T data;
private Node<T> next;
public Node(T data, Node<T> next){
this.data = data;
this.next = next;
}
}
private class LinkedListIterator implements Iterator<T>{
private Node<T> nextNode;
public LinkedListIterator(){
nextNode = head;
}
#Override
public boolean hasNext() {
return (nextNode.next != null);
}
#Override
public T next() {
if(!hasNext()) throw new NoSuchElementException();
T result = nextNode.data;
nextNode = nextNode.next;
return result;
}
}
}
Hey, so I wanted to insert one data after the input data (not index).
I've tried but it always at the end, the data that i want to insert end up at the dront of the link list..
**public static void insertAfter(Object o,Object c){
Node newN = new Node();
Node help = new Node();
Node help2 = new Node();
newN.data = o;
help = head.next;
if(isEmpty()){
head = newN;
newN.next=head;
newN.prev=head;
}
else{
do{
help=help.next;
System.out.println(help);
}while(help.next!=head || !help.data.equals(c));
help2 = help.next;
newN.next = help2;
help2.prev = newN;
help.next=newN;
newN.prev=help;
}**
anyone could help?
thx a bunch!
What are the objects that you are comparing? if they are something other than string than you will have to override equals() method in order to get the correct comparison.
I think you should try another ending condition:
while(help.next!=head && !help.data.equals(c));
By the way, I can only advise you to avoid do...while without serious reasons, and to use getters and setters.
Your code should also be structured diffently. Why are you not writing a private method which just make the insert, i.e. your 5 last lines? Everything would be more readable and reusable.
Also, your variables need clear and meaningful names.
Do it yourself
I begun fixing your solution but ended writing a whole new implementation when wanting to test it... so here goes:
public class DoubleLinkedList<T> {
private class Node {
private Node prev;
private Node next;
private T data;
Node(T data) {
this.data = data;
}
}
Node head;
public boolean isEmpty() {
return head == null;
}
public void insertAfter(T afterThis, T objectToAdd) {
// cannot insert after in a empty list?!
if(isEmpty())
throw new NoSuchElementException("list is empty?");
// find the node where we want to insert the element
Node after = findNodeByObject(afterThis);
// create the node and update the links
addAfter(after, new Node(objectToAdd));
}
private void add(T objectToAdd) {
if (isEmpty()) {
head = new Node(objectToAdd);
head.next = head;
head.prev = head;
}
else {
addAfter(head.prev, new Node(objectToAdd));
}
}
private void addAfter(Node after, Node toAdd) {
Node afterAfter = after.next;
after.next = toAdd;
afterAfter.prev = toAdd;
toAdd.prev = after;
toAdd.next = afterAfter;
}
private Node findNodeByObject(T object) {
Node current = head;
while (true) {
if (current.data.equals(object))
return current;
if (current.next == head)
break;
current = current.next;
}
throw new NoSuchElementException("" + object);
}
#Override
public String toString() {
List<T> printList = new LinkedList<T>();
Node current = head;
while (true) {
printList.add(current.data);
if (current.next == head)
break;
current = current.next;
}
return printList.toString();
}
public static void main(String[] args) throws Exception {
DoubleLinkedList<String> list = new DoubleLinkedList<String>();
list.add("first");
list.add("third");
list.insertAfter("first", "second");
System.out.println(list);
}
}
Extend LinkedList
... and add the insertAfter method like this:
import java.util.LinkedList;
import java.util.ListIterator;
public class MyList<T> extends LinkedList<T> {
private void insertAfter(T first, T second) {
ListIterator<T> iterator = listIterator();
while (iterator.hasNext()) {
if (iterator.next().equals(first)) {
iterator.add(second);
return;
}
}
throw new IndexOutOfBoundsException("Could not find " + first);
}
public static void main(String[] args) throws Exception {
MyList<String> list = new MyList<String>();
list.add("first");
list.add("third");
list.insertAfter("first", "second");
System.out.println(list); // prints "[first, second, third]"
}
}