Priority Queue Introduction and Concept in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA Typescript

const priorityQueue: number[] = [1];

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

A[]

B{}

Cnull

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces {} which create an object, not an array.

Using null or 0 which are not valid initializations for an array.

We use an empty array [] to start a priority queue as a list of numbers.

2fill in blank

medium

Complete the code to add a new element to the priority queue array.

DSA Typescript

priorityQueue.[1](5);

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Apop

Bunshift

Cshift

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop which removes an element instead of adding.

Using shift or unshift which add or remove from the start, not the end.

Use push to add an element to the end of the array, simulating adding to the priority queue.

3fill in blank

hard

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

DSA Typescript

const highestPriority = priorityQueue.[1]();

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Ashift

Bsort

Cslice

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop which removes the last element, not the highest priority.

Using sort or slice which do not remove elements.

After sorting the queue so the smallest number is first, shift removes that first element.

4fill in blank

hard

Fill both blanks to sort the priority queue in ascending order before removing the highest priority element.

DSA Typescript

priorityQueue.[1]((a, b) => a [2] b);

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

Asort

B-

C+

Dreverse

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' in the compare function which sorts incorrectly.

Using reverse which reverses but does not sort.

Use sort with a compare function (a, b) => a - b to sort numbers ascending.

5fill in blank

hard

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

DSA Typescript

function insertAndSort(queue: number[], value: number) {
  queue.[1](value);
  queue.[2]((a, b) => a [3] b);
}

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

Apush

Bsort

C-

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop which removes elements instead of adding.

Using '+' in the compare function which sorts incorrectly.

First push adds the value, then sort with (a, b) => a - b keeps the queue sorted.

Practice

1. What is the main feature that distinguishes a Priority Queue from a regular queue?

easy

A. It does not allow duplicate elements

B. Elements are always dequeued in the order they were inserted

C. Elements are dequeued based on their priority, not just order of insertion

D. It only stores elements of the same value

Solution

1. Step 1: Understand queue behavior

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

2. Step 2: Identify priority queue behavior

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

3. Final Answer:

   Elements are dequeued based on their priority, not just order of insertion -> Option C

4. Quick Check:

   Priority Queue = dequeue by priority [OK]

Hint: Priority queues remove highest priority first, not oldest [OK]

Common Mistakes:

• Confusing priority queue with FIFO queue
• Thinking priority queue sorts elements permanently
• Assuming priority queue disallows duplicates

2. Which of the following is the correct TypeScript syntax to declare a priority queue class with a generic type T?

easy

A. let PriorityQueue = <T>() => { }

B. class PriorityQueue<T> { constructor() {} }

C. function PriorityQueue<T>() { }

D. interface PriorityQueue<T> { new(): T }

Solution

1. Step 1: Recall TypeScript class syntax

   Classes are declared with the class keyword and generics use angle brackets after the class name.

2. Step 2: Check each option

   class PriorityQueue<T> { constructor() {} } correctly declares a generic class with constructor. Others use function, arrow function, or interface incorrectly for class declaration.

3. Final Answer:

   class PriorityQueue<T> { constructor() {} } -> Option B

4. Quick Check:

   Generic class syntax = class Name<T> {} [OK]

Hint: Use class with <T> for generics in TypeScript [OK]

Common Mistakes:

• Using function instead of class for data structure
• Confusing interface with class declaration
• Incorrect generic syntax placement

3. Consider this TypeScript code using a simple priority queue that stores numbers with lower values having higher priority:

class PriorityQueue {
  private data: number[] = [];
  enqueue(value: number) {
    this.data.push(value);
    this.data.sort((a, b) => a - b);
  }
  dequeue() {
    return this.data.shift();
  }
}

const pq = new PriorityQueue();
pq.enqueue(5);
pq.enqueue(1);
pq.enqueue(3);
const first = pq.dequeue();
const second = pq.dequeue();

What are the values of first and second?

medium

A. [5, 1]

B. [5, 3]

C. [3, 5]

D. [1, 3]

Solution

1. Step 1: Trace enqueue operations

   Values 5, 1, 3 are added. After each enqueue, array is sorted ascending: [5], then [1,5], then [1,3,5].

2. Step 2: Trace dequeue operations

   Dequeue removes first element (lowest value): first = 1, second = 3.

3. Final Answer:

   [1, 3] -> Option D

4. Quick Check:

   Sorted ascending, dequeue removes smallest [OK]

Hint: Sort ascending, dequeue removes first element [OK]

Common Mistakes:

• Assuming dequeue removes largest value
• Not sorting after enqueue
• Confusing shift() with pop()

4. The following TypeScript code for a priority queue has a bug. What is the bug?

class PriorityQueue {
  private data: number[] = [];
  enqueue(value: number) {
    this.data.push(value);
  }
  dequeue() {
    return this.data.pop();
  }
}

medium

A. Dequeue removes the last element, not the highest priority

B. Enqueue does not add elements to the queue

C. Data array is not initialized

D. Class is missing a constructor

Solution

1. Step 1: Analyze enqueue method

   Enqueue adds elements to the end of the array correctly.

2. Step 2: Analyze dequeue method

   Dequeue uses pop(), which removes last element, not necessarily highest priority.

3. Final Answer:

   Dequeue removes the last element, not the highest priority -> Option A

4. Quick Check:

   pop() removes last, not priority element [OK]

Hint: Dequeue should remove highest priority, not last element [OK]

Common Mistakes:

• Using pop() instead of shift() or priority removal
• Ignoring sorting or priority logic
• Assuming push() sorts automatically

5. You want to implement a priority queue in TypeScript where higher numbers have higher priority. Which approach correctly maintains the queue so that dequeue() always removes the highest number efficiently?

hard

A. Store elements in an array and sort descending after each enqueue, then remove first element on dequeue

B. Store elements in an array and sort ascending after each enqueue, then remove first element on dequeue

C. Store elements in an unsorted array and remove the maximum element by scanning on each dequeue

D. Store elements in a linked list without sorting and remove the first element on dequeue

Solution

1. Step 1: Understand priority requirement

   Higher numbers have higher priority, so dequeue should remove the largest number.

2. Step 2: Evaluate each approach

   Store elements in an array and sort ascending after each enqueue, then remove first element on dequeue removes the smallest (lowest priority). Store elements in an array and sort descending after each enqueue, then remove first element on dequeue places largest first for efficient removal. Store elements in an unsorted array and remove the maximum element by scanning on each dequeue is correct but inefficient O(n). Store elements in a linked list without sorting and remove the first element on dequeue follows insertion order, not priority.

3. Final Answer:

   Store elements in an array and sort descending after each enqueue, then remove first element on dequeue -> Option A

4. Quick Check:

   Sort descending + remove first = highest priority [OK]

Hint: Sort descending, dequeue removes first for highest priority [OK]

Common Mistakes:

• Sorting ascending but removing wrong end
• Not sorting and scanning entire array on dequeue
• Using linked list without priority order