Question

Implement a priority queue using a DoublyLinkedList where the node with the highest priority (key) is the right-most node.

The remove (de-queue) operation returns the node with the highest priority (key).
If displayForward() displays List (first-->last) : 10 30 40 55
remove() would return the node with key 55.

You will then attach priorityInsert(long key) and priorityRemove() methods). AND Use the provided PQDoublyLinkedTest.java to test your code.

BOTH CODES SHOULD WORK TOGETHER, YOU JUST HAVE TO ADD priorityInsert(int). PLEASE PROVIDE CORRECT CODE.

class Link {
public long dData;
public Link next;
public Link previous;

public Link(long d) {
dData = d;
}

public void displayLink() {
System.out.print(dData + " ");
}
}

class DoublyLinkedList {
private Link first;
private Link last;
public DoublyLinkedList() {
first = null;
last = null;
}

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

public void insertFirst(long dd) {
Link newLink = new Link(dd);

if (isEmpty())
last = newLink;
else
first.previous = newLink;
newLink.next = first;
first = newLink;
}

public void insertLast(long dd) {
Link newLink = new Link(dd);
if (isEmpty())
first = newLink;
else {
last.next = newLink;
newLink.previous = last;
}
last = newLink;
}

public Link deleteFirst() {
Link temp = first;
if (first.next == null)
last = null;
else
first.next.previous = null;
first = first.next;
return temp;
}

public Link deleteLast() {
Link temp = last;
if (first.next == null)
first = null;
else
last.previous.next = null;
last = last.previous;
return temp;
}

public boolean insertAfter(long key, long dd) {
Link current = first;
while (current.dData != key){
current = current.next;
if (current == null)
return false;
}
Link newLink = new Link(dd);
if (current == last) {
newLink.next = null;
last = newLink;
}
else {
newLink.next = current.next;
current.next.previous = newLink;
}
newLink.previous = current;
current.next = newLink;
return true;
}

public Link deleteKey(long key) {
Link current = first;
while (current.dData != key) {
current = current.next;
if (current == null)
return null;
}
if (current == first)
first = current.next;
else
current.previous.next = current.next;
if (current == last)
last = current.previous;
else
current.next.previous = current.previous;
return current;
}

public void displayForward() {
System.out.print("List (first-->last): ");
Link current = first;
while (current != null)
{
current.displayLink();
current = current.next;
}
System.out.println("");
}

public void displayBackward() {
System.out.print("List (last-->first): ");
Link current = last;
while (current != null)
{
current.displayLink();
current = current.previous;
}
System.out.println("");
}

public void insertSorted(long key) {
if (isEmpty() || key < first.dData) {
insertFirst(key);
return;
}

Link current = first;
while (current.next != null) {
if (key >= current.dData && key <= current.next.dData) {
Link lnk = new Link(key);
lnk.next = current.next;
current.next.previous = lnk;
current.next = lnk;
lnk.previous = current;
return;
}
current = current.next;
}
insertLast(key);
}

}

public class PriorityLinkQueue {
private DoublyLinkedList list;
public PriorityLinkQueue() {
list = new DoublyLinkedList();
}

public void insert(long key) {
list.insertSorted(key);
}

public long remove() {
if (list.isEmpty()) {
throw new RuntimeException("Priority Queue is empty!");
}
return list.deleteLast().dData;
}

public boolean isEmpty() {
return list.isEmpty();
}

public void displayForward() {
list.displayForward();
}

  
}

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

class PQDoublyLinkedTest.JAVA ->>> ONLY FOR TESTING, DO NOT CHANGE ANYTHING IN PQDoublyLinkedTest.JAVA

public class PQDoublyLinkedTest
{
public static void main(String[] args)
{ // make a new list
DoublyLinkedList theList = new DoublyLinkedList();

theList.priorityInsert(22); // insert at front
theList.priorityInsert(44);
theList.priorityInsert(66);

theList.priorityInsert(11); // insert at rear
theList.priorityInsert(33);
theList.priorityInsert(55);
  
theList.priorityInsert(10);
theList.priorityInsert(70);
theList.priorityInsert(30);

theList.displayForward(); // display list forward
Link2 removed = theList.priorityRemove();
System.out.print("priorityRemove() returned node with key: ");
removed.displayLink2();
  
} // end main()
} // end class PQDoublyLinkedTest

Answer 1

Here is the completed code for this problem. Comments are included, go through it, learn how things work and let me know if you have any doubts or if you need anything to change. If you are satisfied with the solution, please rate the answer. Thanks

//updated DoublyLinkedList file to include the new insert method (inserts a key in sorted order)

class Link {

public long dData; // data item

public Link next; // next link in list

public Link previous; // previous link in list

// -------------------------------------------------------------

public Link(long d) // constructor

{

dData = d;

}

// -------------------------------------------------------------

public void displayLink() // display this link

{

System.out.print(dData + " ");

}

// -------------------------------------------------------------

} // end class Link

// //////////////////////////////////////////////////////////////

class DoublyLinkedList {

private Link first; // ref to first item

private Link last; // ref to last item

// -------------------------------------------------------------

public DoublyLinkedList() // constructor

{

first = null; // no items on list yet

last = null;

}

// -------------------------------------------------------------

public boolean isEmpty() // true if no links

{

return first == null;

}

// -------------------------------------------------------------

public void insertFirst(long dd) // insert at front of list

{

Link newLink = new Link(dd); // make new link

if (isEmpty()) // if empty list,

last = newLink; // newLink <-- last

else

first.previous = newLink; // newLink <-- old first

newLink.next = first; // newLink --> old first

first = newLink; // first --> newLink

}

// -------------------------------------------------------------

public void insertLast(long dd) // insert at end of list

{

Link newLink = new Link(dd); // make new link

if (isEmpty()) // if empty list,

first = newLink; // first --> newLink

else {

last.next = newLink; // old last --> newLink

newLink.previous = last; // old last <-- newLink

}

last = newLink; // newLink <-- last

}

// -------------------------------------------------------------

public Link deleteFirst() // delete first link

{ // (assumes non-empty list)

Link temp = first;

if (first.next == null) // if only one item

last = null; // null <-- last

else

first.next.previous = null; // null <-- old next

first = first.next; // first --> old next

return temp;

}

// -------------------------------------------------------------

public Link deleteLast() // delete last link

{ // (assumes non-empty list)

Link temp = last;

if (first.next == null) // if only one item

first = null; // first --> null

else

last.previous.next = null; // old previous --> null

last = last.previous; // old previous <-- last

return temp;

}

// -------------------------------------------------------------

// insert dd just after key

public boolean insertAfter(long key, long dd) { // (assumes non-empty list)

Link current = first; // start at beginning

while (current.dData != key) // until match is found,

{

current = current.next; // move to next link

if (current == null)

return false; // didn't find it

}

Link newLink = new Link(dd); // make new link

if (current == last) // if last link,

{

newLink.next = null; // newLink --> null

last = newLink; // newLink <-- last

} else // not last link,

{

newLink.next = current.next; // newLink --> old next

// newLink <-- old next

current.next.previous = newLink;

}

newLink.previous = current; // old current <-- newLink

current.next = newLink; // old current --> newLink

return true; // found it, did insertion

}

// -------------------------------------------------------------

public Link deleteKey(long key) // delete item w/ given key

{ // (assumes non-empty list)

Link current = first; // start at beginning

while (current.dData != key) // until match is found,

{

current = current.next; // move to next link

if (current == null)

return null; // didn't find it

}

if (current == first) // found it; first item?

first = current.next; // first --> old next

else

// not first

// old previous --> old next

current.previous.next = current.next;

if (current == last) // last item?

last = current.previous; // old previous <-- last

else

// not last

// old previous <-- old next

current.next.previous = current.previous;

return current; // return value

}

// -------------------------------------------------------------

public void displayForward() {

System.out.print("List (first-->last): ");

Link current = first; // start at beginning

while (current != null) // until end of list,

{

current.displayLink(); // display data

current = current.next; // move to next link

}

System.out.println("");

}

// -------------------------------------------------------------

public void displayBackward() {

System.out.print("List (last-->first): ");

Link current = last; // start at end

while (current != null) // until start of list,

{

current.displayLink(); // display data

current = current.previous; // move to previous link

}

System.out.println("");

}

/**

* method to insert a key in sorted order

*

* @param key

* key to be added

*/

public void insertSorted(long key) {

// if list is empty or key is less than current first, inserting at

// first

if (isEmpty() || key < first.dData) {

insertFirst(key);

return; // exiting method

}

// taking a reference to first

Link current = first;

// looping as long as current.next is not null

while (current.next != null) {

// checking if key can be added between current and current.next

if (key >= current.dData && key <= current.next.dData) {

// adding between current and current.next and updating all

// links

Link lnk = new Link(key);

lnk.next = current.next;

current.next.previous = lnk;

current.next = lnk;

lnk.previous = current;

return; //exiting

}

//otherwise, advancing to next link

current = current.next;

}

//if the element is still not inserted, adding to the end

insertLast(key);

}

// -------------------------------------------------------------

} // end class DoublyLinkedList

// PriorityLinkQueue.java (representing a Priority Queue using the above doubly linked list)

//class representing a PriorityLinkQueue

public class PriorityLinkQueue {

// using a DoublyLinkedList as instance variable

private DoublyLinkedList list;

// constructor initializing empty pq

public PriorityLinkQueue() {

list = new DoublyLinkedList();

}

// inserts a key in proper position

public void insert(long key) {

list.insertSorted(key);

}

// removes and returns the element with highest value

public long remove() {

// if empty, throwing an exception

if (list.isEmpty()) {

throw new RuntimeException("Priority Queue is empty!");

}

// otherwise, removing last value from doubly list and returning the

// data

return list.deleteLast().dData;

}

// returns true if pq is empty

public boolean isEmpty() {

return list.isEmpty();

}

// displays the queue forward

public void displayForward() {

list.displayForward();

}

// main method for testing the new PriorityLinkQueue, move this code to

// anywhere else if you want to.

public static void main(String[] args) {

// creating a queue

PriorityLinkQueue q = new PriorityLinkQueue();

// adding 10 random keys between 0 and 99 to the queue, displaying it

// after every insertion

for (int i = 0; i < 10; i++) {

q.insert((long) (Math.random() * 100));

q.displayForward();

}

// looping and removing all entries from queue, displaying the queue

// after every removal

while (!q.isEmpty()) {

System.out.println(q.remove() + " is removed");

q.displayForward();

}

}

}

/*OUTPUT*/

List (first-->last): 39

List (first-->last): 4 39

List (first-->last): 4 9 39

List (first-->last): 4 9 14 39

List (first-->last): 4 5 9 14 39

List (first-->last): 4 5 9 13 14 39

List (first-->last): 4 5 9 13 14 39 74

List (first-->last): 4 5 9 13 14 39 74 95

List (first-->last): 4 5 9 13 14 39 74 90 95

List (first-->last): 4 5 9 13 14 36 39 74 90 95

95 is removed

List (first-->last): 4 5 9 13 14 36 39 74 90

90 is removed

List (first-->last): 4 5 9 13 14 36 39 74

74 is removed

List (first-->last): 4 5 9 13 14 36 39

39 is removed

List (first-->last): 4 5 9 13 14 36

36 is removed

List (first-->last): 4 5 9 13 14

14 is removed

List (first-->last): 4 5 9 13

13 is removed

List (first-->last): 4 5 9

9 is removed

List (first-->last): 4 5

5 is removed

List (first-->last): 4

4 is removed

List (first-->last):