Using Java only.

Implement Queue using Linked List$$A queue works in a first$-$in$-$first$-$out fashion $($see sections $3\mathrm{.}15$ and $3\mathrm{.}16$ for details$).$ You will implement the following methods using Linked List methods made available to you:

$*$ public void enqueue$()$which adds an element at the end of the queue.

$*$ public int dequeue$()$which removes the first element from the queue and returns it$.$

$*$ public int peek$()$which returns the first element without removing it from the queue.

Sample Input$/$Output

$4981253616-1$

should output the following:

Queue elements:

$4981253616$

Peek returns: $49$

After popping twice:

$253616$

Input: $-1$

Output:

Queue elements:

Empty.

Peek returns: $-1$

After popping twice:

Empty.

Input: $12345-1$

Output:

Your output

Queue elements:

$12345$

Peek returns: $1$

After popping twice:

$345$

Input: $2040-1$

Output:

Your output

Queue elements:

$2040$

Peek returns: $20$

After popping twice:

Empty.

public class Node$\{$

int data;

Node next;

public Node$($int data$)\{$

this.data $=$ data;

next $=$ null;

$\}$

$\}$

import java.util.Scanner;

public class LinkedListBasic $\{$

Node head;

Node tail;

$//$ inserts data to the end of the list only using the head pointer

public void append$($int data$)\{$

if$($head $==$ null$)\{$

Node newNode $=$ new Node$($data$)$;

head $=$ newNode;

$\}$ else $\{$

Node newNode $=$ new Node$($data$)$;

Node currentNode $=$ head;

while$($currentNode$.$next $!=$ null$)\{$

currentNode $=$ currentNode.next;

$\}$

currentNode.next $=$ newNode;

$\}$

$\}$

$//$ inserts data to the beginning of the list

public void prepend$($int data$)\{$

if$($head $==$ null$)\{$

Node newNode $=$ new Node$($data$)$;

head $=$ newNode;

return;

$\}$

Node newNode $=$ new Node$($data$)$;

newNode.next $=$ head;

head $=$ newNode;

$\}$

$//$ insert a new data after the given one

public void insertAfter$($int givenData, int newData$)\{$

if$($head $==$ null$)\{$

Node newNode $=$ new Node$($newData$)$;

head $=$ newNode;

return;

$\}$ else$\{$

Node currentNode $=$ head;

while$($currentNode$.$next $!=$ null$)\{$

if$($currentNode$.$data $==$ givenData$)\{$

Node newNode $=$ new Node$($newData$)$;

newNode.next $=$ currentNode.next;

currentNode.next $=$ newNode;

$\}$

$\}$

$\}$

$\}$

$//$ check if the list is empty

public boolean isEmpty$()\{$

return $($head $==$ null$)$;

$\}$

$//$ prints the list

public void print$()\{$

if$($isEmpty$())\{$

System.out.println$("$Empty$.")$;

$\}$

Node currentNode $=$ head;

while$($currentNode $!=$ null$)\{$

System.out.printf$("\%$d $",$ currentNode.data$)$;

currentNode $=$ currentNode.next;

$\}$

$\}$

public Node removeFirst$()\{$

Node first $=$ head;

head $=$ head.next;

return first;

$\}$

public void removeLast$()\{$

if$($head $==$ null$)\{$

return;

$\}$

Node current $=$ head;

while$($current$.$next.next $!=$ null$)\{$

current $=$ current.next;

$\}$

current.next $=$ null;

$\}$

$\}$

import java.util.Scanner;

public class QueueList $\{$

LinkedListBasic queueList $=$ new LinkedListBasic$()$;

$//$ Adds new elements to the end of the queue

public void enqueue$($int data$)\{$

$//$ Complete this method

$\}$

$//$ Removes the front element from the queue

public void dequeue$()\{$

$//$ Complete this method

$\}$

$//$ returns the front element of the queue but does not remove it$.$

public int peek$()\{$

$//$ Complete this method

$\}$

public void print$()\{$

queueList.print$()$;

$\}$

public static void main$($String$[]$ args$)\{$

QueueList queue $=$ new QueueList$()$;

Scanner input $=$ new Scanner$($System$.$in$)$;

while $($true$)\{$

int data $=$ input.nextInt$()$;

if $($data $==-1)$

break;

queue.enqueue$($data$)$;

$\}$

$//$ printing queue elements

System.out.println$("$Queue elements:"$)$;

queue.print$()$;

$//$ printing the value peek returns

int peek$\_$item $=$ queue.peek$()$;

System.out.printf$("\%$nPeek returns: $\%$d$\%$n$",$ peek$\_$item$)$;

$//$ printing queue elements after pop

queue.dequeue$()$;

queue.dequeue$()$;

System.out.println$("$

After popping twice:"$)$;

queue.print$()$;

$\}$

$\}$