Priority Queue Introduction and Concept in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an empty priority queue using an array.

DSA Javascript

const priorityQueue = [1];

Drag options to blanks, or click blank then click option'

A[]

B{}

Cnull

Dnew Set()

Attempts:

3 left

💡 Hint

Common Mistakes

Using an object {} instead of an array.

Using null which is not a data structure.

Priority queues can be implemented using arrays. Here, an empty array [] initializes the queue.

2fill in blank

medium

Complete the code to add an element with priority to the priority queue array.

DSA Javascript

priorityQueue.push({value: 'task1', priority: [1]);

Drag options to blanks, or click blank then click option'

A1

B'high'

C'low'

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using strings like 'high' or 'low' instead of numbers.

Using boolean values.

Priority is usually represented by a number. Lower numbers often mean higher priority. Here, 1 is used as a priority.

3fill in blank

hard

Fix the error in the code to remove the highest priority element (lowest number) from the queue.

DSA Javascript

priorityQueue.sort((a, b) => a.priority [1] b.priority);
const highestPriority = priorityQueue.shift();

Drag options to blanks, or click blank then click option'

A!=

B>

C===

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using > sorts descending, which is wrong here.

Using === or != causes incorrect sorting.

Sorting ascending by priority means smaller numbers come first. Use < to sort from lowest to highest.

4fill in blank

hard

Fill both blanks to check if the priority queue is empty and print a message.

DSA Javascript

if (priorityQueue.[1] === [2]) {
  console.log('Queue is empty');
}

Drag options to blanks, or click blank then click option'

Alength

B0

Csize

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using size which is for sets/maps, not arrays.

Comparing length to null instead of 0.

Arrays use length to check how many elements they have. If length is 0, the queue is empty.

5fill in blank

hard

Fill all three blanks to create a function that inserts an element and keeps the queue sorted by priority.

DSA Javascript

function enqueue(queue, element) {
  queue.push(element);
  queue.sort((a, b) => a.[1] [2] b.[3]);
}

Drag options to blanks, or click blank then click option'

Apriority

B<

C>

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Sorting by value instead of priority.

Using > which sorts descending.

The function sorts the queue by the priority property in ascending order using <.

Practice

1. What is the main difference between a Priority Queue and a regular queue?

easy

A. Elements are always dequeued in the order they were enqueued

B. Elements are dequeued based on priority, not just order of arrival

C. It only stores numbers, not other data types

D. It uses a stack internally to manage elements

Solution

1. Step 1: Understand queue behavior

   A regular queue removes elements in the order they arrive (FIFO).

2. Step 2: Understand priority queue behavior

   A priority queue removes elements based on their priority, not just arrival order.

3. Final Answer:

   Elements are dequeued based on priority, not just order of arrival -> Option B

4. Quick Check:

   Priority Queue = priority-based removal [OK]

Hint: Priority queue removes highest priority first, not oldest [OK]

Common Mistakes:

• Thinking priority queue works like a normal queue
• Assuming priority queue only stores numbers
• Confusing priority queue with stack

2. Which of the following is the correct way to declare a priority queue class in JavaScript?

easy

A. let PriorityQueue = () => { this.items = []; }

B. function PriorityQueue() { this.items = {}; }

C. class PriorityQueue { constructor() { this.items = []; } }

D. var PriorityQueue = new Array();

Solution

1. Step 1: Check class syntax

   class PriorityQueue { constructor() { this.items = []; } } uses correct ES6 class syntax with constructor initializing an array.

2. Step 2: Check other options

   function PriorityQueue() { this.items = {}; } uses function but initializes an object, not array; let PriorityQueue = () => { this.items = []; } uses arrow function incorrectly for class; var PriorityQueue = new Array(); assigns array directly, not a class.

3. Final Answer:

   class PriorityQueue { constructor() { this.items = []; } } -> Option C

4. Quick Check:

   Correct class syntax = class PriorityQueue { constructor() { this.items = []; } } [OK]

Hint: Use ES6 class with constructor for priority queue [OK]

Common Mistakes:

• Using arrow functions as classes
• Initializing items as object instead of array
• Assigning array directly without class

3. Consider this JavaScript code using a simple priority queue implementation:

class PriorityQueue {
  constructor() {
    this.items = [];
  }
  enqueue(element, priority) {
    this.items.push({element, priority});
    this.items.sort((a, b) => a.priority - b.priority);
  }
  dequeue() {
    return this.items.shift().element;
  }
}

const pq = new PriorityQueue();
pq.enqueue('A', 2);
pq.enqueue('B', 1);
pq.enqueue('C', 3);
console.log(pq.dequeue());

What will be printed?

medium

A. 'B'

B. 'A'

C. 'C'

D. undefined

Solution

1. Step 1: Analyze enqueue operations

   Elements are added with priorities: A(2), B(1), C(3). After each enqueue, array is sorted by priority ascending.

2. Step 2: Analyze dequeue operation

   Dequeue removes first element (lowest priority number). After sorting, order is B(1), A(2), C(3). So dequeue returns 'B'.

3. Final Answer:

   'B' -> Option A

4. Quick Check:

   Lowest priority dequeued first = 'B' [OK]

Hint: Lowest priority number dequeued first [OK]

Common Mistakes:

• Assuming FIFO order instead of priority
• Confusing priority sorting order
• Forgetting to return element, not object

4. The following code is intended to implement a priority queue, but it has a bug:

class PriorityQueue {
  constructor() {
    this.items = [];
  }
  enqueue(element, priority) {
    this.items.push({element, priority});
  }
  dequeue() {
    return this.items.shift().element;
  }
}

What is the main problem with this code?

medium

A. It stores elements as objects instead of arrays

B. It uses shift() instead of pop() to remove elements

C. It does not initialize items array in constructor

D. It does not sort items by priority before dequeue

Solution

1. Step 1: Check enqueue method

   Enqueue adds elements but does not sort by priority.

2. Step 2: Check dequeue method

   Dequeue removes first element, but without sorting, priority order is not guaranteed.

3. Final Answer:

   It does not sort items by priority before dequeue -> Option D

4. Quick Check:

   Missing sorting causes wrong dequeue order [OK]

Hint: Sort items by priority before dequeue [OK]

Common Mistakes:

• Assuming shift() removes highest priority
• Ignoring sorting requirement
• Confusing object vs array storage

5. You want to implement a priority queue that supports fast insertion and removal of the highest priority element. Which data structure is best suited for this, and why?

hard

A. Binary heap, because it provides efficient insert and remove operations

B. Linked list, because it maintains insertion order

C. Stack, because it removes the last inserted element quickly

D. Array, because it allows easy sorting after each insertion

Solution

1. Step 1: Understand operation needs

   Priority queue needs fast insertion and removal of highest priority element.

2. Step 2: Compare data structures

   Array requires sorting after insertion (slow). Linked list maintains order but slow for priority. Stack removes last inserted only. Binary heap supports insert and remove in O(log n) time efficiently.

3. Final Answer:

   Binary heap, because it provides efficient insert and remove operations -> Option A

4. Quick Check:

   Binary heap = efficient priority queue [OK]

Hint: Use binary heap for efficient priority queue operations [OK]

Common Mistakes:

• Choosing array and ignoring sorting cost
• Confusing stack with priority queue
• Ignoring time complexity of operations