Manual Linked List - Insert before method - java

This is part of an exercise we did in class, I just can't figure it out...
The required method is the insertBefore(object data) method wherein when a user choose this method it will prompt it to enter a data to be inserted before the reference data(input by user)
An example of how it should run:
// assuming linked list has data 1,2,3 inserted already
Choose Method:
1)Insert Before
choice: 1 // input by user
====Insert Before====
Enter Reference data: 2 // input by user
Enter Data to be inserted: 5 // input by user
supposed output: 1,5,2,3
Here's my code for this exercise: (This is inside a class called LinkList with variables
protected int end;
protected Node start;
and an inner class called Node)
private class Node
{
public char data;
public Node next;
public Node(char data)
{
this.data = data;
}
}
public void insertBef(char ref,char data)
{
Node temp = new Node(data);
Node current = start;
if(end!=0)
{
for(int i = 1; i<end; i++)
{
if(current == start)
{
Node newNode = start;
newNode.data = current.data;
newNode.next = temp;
current = current.next;
}
else if(current.data == ref)
{
Node newNode = current;
newNode.data = current.data;
newNode.next = temp;
current = current.next;
}
}
end++;
}
else
{
temp.next = start;
start = temp;
}
end++;
}
But when I run my code it ouputs 3,5, not 1,5,2,3! I can't see where I might have gone wrong...
Can someone please tell me where the mistake is, and explain how I might fix it?
I understand that to be able to insert before a reference value you should:
Make a new node for the new data
Make a temporary node for the reference value and link
Make the link of the data before the reference value point to the new node and make the link of the new node point to the temporary node
I just can't seem to figure out how to implement it in Java code...

When programming, if it seems hard, you're probably going about it the wrong way...
You only need one line of code to accomplish the task!
list.add(list.indexOf(reference), data);
Here's this line wrapped as an insertBefore method:
public static void insertBefore(List<Integer> list, int reference, int data) {
list.add(list.indexOf(reference), data);
}
Here's a test using your example:
public static void main(String[] args) {
List<Integer> list = new ArrayList<Integer>(Arrays.asList(1, 2, 3));
insertBefore(list, 2, 5);
System.out.println(list);
}
Output:
[1, 5, 2, 3]
Note: This code will throw an exception if the reference element is not found.
I'll leave it to you to plug that hole.

First things first: for loops are generally a bad idea with linked lists. while loops are much better; you can do something like while(next != null).
Looking at your code, you seem to have a few problems.
Your for loop is overly complicated, and doesn't seem to make a lot of sense. Here's how your loop should look:
Get head node
Begin looping through the list, checking the next node's value as you go
Once you find that the next node's value is the one you're looking for, creating a new node.
Insert the new node by setting its Next value to be equal to the current node's Next value, then set the current node's next value to be the new node.
Return from the function.
Your middle bullet point is actually unnecessary, and I have no idea what you use end for. Regardless, you seem to have the basic principle down, so I won't feel like I'm spoiling you by posting code.
Now, I'm not sure what your start is. Does it hold a value, or is it a dedicated head node? I'd vote for a dedicated head node, I generally find it easier to work with because you don't need to add code for a special case where the number should come before the head. So your start node should be "empty"; the value it holds is ignored, the only thing it's used for is to keep a pointer to the first legit node in the list. If you do this, the insertBef method becomes incredibly simple. NOTE: untested code to follow.
public void insertBef(char ref, char data)
{
Node current = start;
while( current.next != null )
{
if( current.next.value == ref )
{
Node n = new Node(data);
n.next = current.next;
current.next = n;
return;
}
current = current.next;
}
}
Please don't just copy the code. If you have questions, post them and I'll do my best to answer.

Related

Constructor Node cannot be applied to given types while working with nodes

I am working on a code that puts new elements on MyStack if they are unique. I had to copy and paste the node starting code, so I'm having a bit of trouble with an issue. I keep getting two error messages, even after trying various workarounds and I'm not really understanding why. I've even tried using some helper functions I've previously made that have worked before so I'm extra confused.
The two errors I consistently get are:
-cannot infer type arguments for MyStack.Node (actual and formal arguments differ in length)
-constructor node cannot be applied to given types. Required, no arguments, found: anything,
Here's my code:
public class MyStack<Anything>
{
private Node first, last;
private class Node<Anything>
{
Anything item;
Node next;
}
public boolean contains(Anything value)
{
for (Node curr = first; curr != null; curr = curr.next)
{
if (value.equals(curr.item)) {
return true;
}
}
return false;
}
public void add(Anything value)
//method that adds a new value to the end of the list
//COMPLETE
{
Node temp = first;
while(temp.next!=null){ //finds the end
temp=temp.next;
}
temp.next=new Node(value, null); //assigns new value
}
public void enqueue(Anything info){
if (this.contains(info)==true) { //if the info is already present
System.out.println("the stack already contains this value");
return;
}
//if we actually need to add the info
if (first == null) { //if there is nothing in the stack
Node temp= first;
first = new Node<>(info,temp);
first = temp;
return;
}
if (first != null) { //if there is already stuff
Node temp = first;
while (temp.next == null)
{ Node newNode= new Node<>(info, temp);
temp.next = newNode;
}
return;
}
}
}
As #Andreas already pointed out, Node needs a constructor.
There are a few other flaws in your Code:
Use Generics
With your Code, you can only store Objects of the class Anything, what strongly limits its reusability. Use a generic instead and you can reuse this class for many more purposes.
Linked List
I suggest, you use the paradigm of a double-linked-list. That way you do not need to find the last Node to add something to the Stack. Node now has a pointer to its previous and next element.
Use the last Object
You have the object last but never use it. To find out, whether the current object is the last one you compare the value to null. This has the effect, that storing a null value will break your List. Instead compare to the Object last, this object is unique and guarantees you, that you are at the end of the list. Both first and last are Nodes that do not contain a value and are simply used to mark the start/end of your List.
Adding elements
Using the changes above, the code in the Method enqueue(T value) becomes significantly simpler: You just check whether contains(value) and decide whether you add the value to the List or not.
All these changes applied result in following code:
public class MyStack<T extends Object> {
private Node first, last;
public MyStack() {
first = new Node(null, null, null);
last = new Node(null, null, first);
first.next = last;
}
private class Node {
T item;
Node next;
Node previous;
public Node(T item, Node next, Node previous) {
this.item = item;
this.next = next;
this.previous = previous;
}
}
public boolean contains(T value) {
for (Node curr = first.next; curr != last; curr = curr.next) {
if (value.equals(curr.item)) {
return true;
}
}
return false;
}
/**
* method that adds a new value to the end of the list
*/
public void add(T value)
{
Node secondLast = last.previous;
Node added = new Node(value, last, secondLast);
secondLast.next = added;
last.previous = added;
}
/**
* only adds value if it is not already contained by the Stack
*/
public void enqueue(T value) {
if (this.contains(value) == true) { // if the info is already present
System.out.println("the stack already contains this value");
}
else {
add(value);
}
}
public static void main(String[] args) {
MyStack<String> test = new MyStack<>();
test.add("foo");
test.add("bar");
test.add("baz");
System.out.println(test.contains("bar"));
System.out.println(test.contains("new"));
test.enqueue("baz");
test.enqueue("MyStack");
}
}
Naming
As you may have noticed, in my explanation I called this class a List. This is because it fulfills more of the characteristics of a List. A Stack usually only provides the methods push to put something at the top of the Stack and pop to remove and return the topmost Object. Optionally peek can return the topmost Object, without removing it from the Stack.
Also consider renaming the method enqueue: enqueue is used in Queues (obviously) and Queues do not forbid to add two equal Objects. So the name is misleading. I would call this method something like addIfNotContaining.
In my Opinion you should name this class to be a List and add a method get(int i) to get a specific element at a position. Naturally adding some other methods like size ect. to comply with a standard List. But I assume you already had, but did not post them because they are not related to your problem.
Multithreading
This Class is far from threadsave. But I let you figure out yourself how to make it threadsave if needed.

How to find the first value in a linked list?

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!
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.
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
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);
}
}
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;
}

Recursively delete the last occurrence in a linked list, Java

class Link{
private int value;
private Link next;
}
I am asked to write a recursive method to delete last occurrence of a certain value, say 4.
before 2->3->4->5->4->2
after 2->3->4->5->2
The last occurrence only. I know how to delete all occurrence but I can't tell if its the last occurrence. No helper method is allowed.
The one to delete all occurrence
public Link deleteAll(){
if (next == null){
return value==4ļ¼Ÿ null:this;
}else{
if (value == 4){
return next.deleteAll();
}
next = next.deleteAll();
return this;
}
}
You can declare a pointer to the last occurred node and delete that node when reached the last element in list. Following steps explains that -
Declare two pointers one is next as in your above code another can be temp.
Iterate through list using next like you doing in deleteAll method above.
If you find the node you looking for assign that node to temp.In your case 4.
When next is null you reached the end of list now delete, whatever node is in temp delete that node. If temp is still null than no node found in given key.
EDIT:
Possible pseudo Code in case of recursion:
public void deleteLast(Node node,Node temp,Node prev, int data)
{
if(node==null)
{
if(temp!=null && temp.next.next!=null){
temp.next = temp.next.next;}
if(temp.next.next==null)
temp.next = null;
return;
}
if(node.data==data)
{
temp = prev;
}
prev = node;
deleteLast(node.next, temp, prev, int data);
}
Above code should be able to solve your problem. I made some edit in my approach which should be obvious from the code but let me describe it below
I added a prev pointer. Because if we want to delete a particular node we need to assign its next to prev node's next.So, we need the prev node not the node that we want to delete.
I think this change will follow in iterative approach too.
Not really answering your exact question, but as an alternative option, you might consider the following.
Write a recursive method to delete the first occurrence of a specified value, something like this:
public Link deleteFirst(int target) {
if (value == target) {
return next;
}
next = (next == null) ? null : next.deleteFirst(target);
return this;
}
Then you could write a reverse() method as either an iterative or recursive method as you see fit. I haven't included this, but googling should show some useful ideas.
Finally the method to remove the last occurrence of a value from the linked list could then be written like this:
public Link deleteLast(int target) {
return reverse().deleteFirst(target).reverse();
}
Note that as long as your reverse() method is linear complexity, this operation will be linear complexity as well, although constants will be higher than necessary.
The trick is to do the work on the way back -- there is no need for additional parameters, helpers or assumptions at all:
Link deleteLast(int target) {
if (next == null) {
return null;
}
Link deleted = next.deleteLast(target);
if (deleted == null) {
return value == target ? this : null;
}
if (deleted == next) {
next = deleted.next;
}
return deleted;
}

Circular Linked List Node Insertion Java

I tried implementing the insert method for circular linked list. I think I had some success.
Problem:
When I display the list. The display method will loop because every next variable of the link is linked to a non-null node object. So head will never be a null object. From what I recall about singly linked list, head always point to the first node in the list or the first node with data inside of it.
My conceptual understanding of circular linked list:
From what I can understand circular linked is somewhat like a singly linked list but with a slight twist: the next variable of the tail object points to the head.
I'm coding it like the diagram has presented provided by the source link.
Source: http://sourcecodemania.com/circular-linked-lists/
public void insert(String data)
{
Link link = new Link(data);
if(head == null)
{
head = link;
tail= link;
}
else
{
tail.next = link;
tail = link;
tail.next = head;
}
}
public void display()
{
// good implementation for display #2
while(head != null)
{
// System.out.println (head.data);
head = head.next;
}
}
Once you insert at least one element, you would never come across null. It will keep on going till infinity.
Also, it might not be a good idea to modify head just for displaying the list. Operation like display should not have any side effects.
In stead, keep a member field size in your list class and update it in each insert and delete method.
Now you would know how many times you should iterate the loop.
ListClassName current = head; // Head is not modified.
for (int i = 0; i < this.size; i++) {
// System.out.println (current.data);
current = current.next;
}
Good luck.
You can keep a reference to the first Link object and check to make sure head is not equal to this object while looping:
public void display()
{
boolean first=true;
Link firstItem=null;
// good implementation for display #2
while(head != null && head!= firstItem)
{
if(first){
firstItem=head;
first=false;
}
// System.out.println (head.data);
head = head.next;
}
}

Implement an algorithm to delete a node in the middle of a single linked list, given only access to that node

Example input: the node ā€˜cā€™ from the linked list a->b->c->d->e Result: nothing is returned, but the new linked list looks like a->b->d->e
I do understand that ppl have already asked this question before, but since my reputation is not high enough yet, I couldn't ask my question in that thread. so here goes my quesetion:
So in the solution, when deleting the middle node we do:
public static boolean deleteNode(LinkedListNode n) {
if (n == null || n.next == null) {
return false; // Failure
}
LinkedListNode next = n.next;
n.data = next.data;
n.next = next.next;
return true;
}
But what I don't understand is that why can't I just do n = next?
It is probably a trivial question, but I didn't seem to find a good explanation for this question
If you just do n = next then you have only changed what object your local reference variable n refers to; you haven't modified any part of the list.
The trick to "deleting" the current node is to overwrite it with the next one:
n.data = next.data;
n.next = next.next;
Now you are modifying fields of the object that is referred to by n, which is a part of the actual list.
In C++, the code you wrote would look like this:
bool deleteNode(LinkedListNode* n) {
if (n == null || (*n).next == null) {
return false; // Failure
}
LinkedListNode* next = (*n).next;
(*n).data = (*next).data;
(*n).next = (*next).next;
return true;
}
So what does that mean? When you call this method, in C++ it would look like this:
LinkedListNode* listNode = new LinkedListNode();
deleteNode(&listNode);
This is important, because that means you're just sending an address over, and not the entire object. This means that you don't actually have access to the node you gave as a parameter to the method, you only have a reference to its address.
Basically, in Java, you can't do the following C++ code:
*n = *next;
You can't modify the listNode object that's outside of the method. You only get its address. And you are only modifying the copy of its address, not the address itself.
Basically, it's because in Java, the pointer of the class is passed by value (as a copy), and primitives are also passed by value (as a copy).
somehow, it overwrites the current node which is supposed to be deleted with the
data of next node to it,and delete the next node.
LinkedListNode next = n.next;
n.data = next.data;
n.next = next.next;
That is how the code comes.

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