Priority Queue Introduction and Concept in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a priority queue of integers.

DSA C#

PriorityQueue<int, int> pq = new PriorityQueue<int, int>();
pq.Enqueue(10, [1]);

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

A10

B1

C5

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using the item value as priority instead of a separate priority number.

Using a priority of 0 which is valid but less clear here.

The priority value determines the order. Lower priority values come out first. Here, 1 is used as priority.

2fill in blank

medium

Complete the code to dequeue the highest priority item from the priority queue.

DSA C#

int highestPriorityItem = pq.[1]();

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

APeek

BPop

CDequeue

DRemove

Attempts:

3 left

💡 Hint

Common Mistakes

Using Peek() which does not remove the item.

Using incorrect method names like Pop or Remove.

Dequeue() removes and returns the item with the highest priority (lowest priority number).

3fill in blank

hard

Fix the error in the code to correctly enqueue an item with priority.

DSA C#

pq.Enqueue([1], 2);

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

A"apple"

Bapple

C10

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Not using quotes for string literals.

Passing null which is invalid here.

Enqueue expects the item as a string or int. "apple" is a string literal, apple without quotes is undefined.

4fill in blank

hard

Fill both blanks to create a priority queue and enqueue an item with priority 3.

DSA C#

var pq = new PriorityQueue<[1], [2]>();
pq.Enqueue("task", 3);

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

Astring

Bint

Cfloat

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using float or bool as priority type which is uncommon.

Using int as item type when item is a string.

The priority queue stores items of type string and priorities of type int.

5fill in blank

hard

Fill all three blanks to dequeue the highest priority item and print it.

DSA C#

var item = pq.[1]();
Console.[2](item);
// PriorityQueue stores items of type [3]

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

ADequeue

BWriteLine

Cstring

DPeek

Attempts:

3 left

💡 Hint

Common Mistakes

Using Peek instead of Dequeue.

Using Console.Write instead of WriteLine.

Incorrect item type in comment.

Dequeue() removes the highest priority item, WriteLine prints it, and the item type is string.

Practice

1. What is the main characteristic of a Priority Queue compared to a regular queue?

easy

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

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

C. It only stores elements of the same data type

D. It allows random access to elements by index

Solution

1. Step 1: Understand queue behavior

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

2. Step 2: Understand priority queue behavior

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

3. Final Answer:

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

4. Quick Check:

   Priority queue = priority-based removal [OK]

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

Common Mistakes:

• Confusing priority queue with FIFO queue
• Thinking priority queue allows random access
• Assuming priority queue only stores one data type

2. Which of the following is the correct way to declare a priority queue of integers in C# using System.Collections.Generic.PriorityQueue?

easy

A. var pq = new PriorityQueue();

B. var pq = new PriorityQueue<int>();

C. PriorityQueue<int> pq = new PriorityQueue<>();

D. PriorityQueue pq = new PriorityQueue();

Solution

1. Step 1: Recall PriorityQueue syntax in C#

   The generic PriorityQueue requires two types: element type and priority type.

2. Step 2: Check each option

   var pq = new PriorityQueue(); correctly uses PriorityQueue<int, int>. Others have syntax errors or missing types.

3. Final Answer:

   var pq = new PriorityQueue(); -> Option A

4. Quick Check:

   PriorityQueue<TElement, TPriority> syntax [OK]

Hint: PriorityQueue needs element and priority types in angle brackets [OK]

Common Mistakes:

• Omitting priority type in generic declaration
• Using wrong constructor syntax
• Trying to declare without generic types

3. What will be the output of the following C# code?

var pq = new PriorityQueue();
pq.Enqueue("apple", 3);
pq.Enqueue("banana", 1);
pq.Enqueue("cherry", 2);
while (pq.Count > 0) {
    Console.Write(pq.Dequeue() + " ");
}

medium

A. banana cherry apple

B. apple cherry banana

C. cherry banana apple

D. apple banana cherry

Solution

1. Step 1: Understand enqueue priorities

   Elements are enqueued with priorities: apple=3, banana=1, cherry=2.

2. Step 2: Dequeue order based on priority

   PriorityQueue dequeues elements with lowest priority first: banana(1), cherry(2), apple(3).

3. Final Answer:

   banana cherry apple -> Option A

4. Quick Check:

   Lowest priority dequeued first = banana cherry apple [OK]

Hint: PriorityQueue dequeues lowest priority first by default [OK]

Common Mistakes:

• Assuming highest priority dequeued first
• Mixing element order with priority order
• Ignoring priority values

4. Identify the error in the following C# code using PriorityQueue:

var pq = new PriorityQueue();
pq.Enqueue(10, 2);
pq.Enqueue(5);
pq.Enqueue(20, 1);

medium

A. Enqueue method does not exist

B. PriorityQueue cannot store integers

C. PriorityQueue requires three generic types

D. Missing priority argument in second Enqueue call

Solution

1. Step 1: Check Enqueue method parameters

   Enqueue requires two arguments: element and priority.

2. Step 2: Identify incorrect call

   The second Enqueue call pq.Enqueue(5); misses the priority argument.

3. Final Answer:

   Missing priority argument in second Enqueue call -> Option D

4. Quick Check:

   Enqueue needs element and priority [OK]

Hint: Enqueue always needs element and priority arguments [OK]

Common Mistakes:

• Forgetting priority argument in Enqueue
• Thinking PriorityQueue needs three types
• Assuming Enqueue accepts one argument

5. You want to process tasks with priorities where higher numbers mean higher priority. How can you use C#'s PriorityQueue to dequeue tasks in order from highest to lowest priority?

hard

A. Use PriorityQueue as is; it dequeues highest priority first

B. Sort the queue manually after enqueueing

C. Store priorities as negative values when enqueuing

D. Use a Stack instead of PriorityQueue

Solution

1. Step 1: Understand PriorityQueue default behavior

   C# PriorityQueue dequeues elements with lowest priority first by default.

2. Step 2: Adjust priorities to reverse order

   To get highest priority first, store priorities as negative values (e.g., -priority).

3. Final Answer:

   Store priorities as negative values when enqueuing -> Option C

4. Quick Check:

   Invert priorities to reverse dequeue order [OK]

Hint: Invert priorities (negative) to get highest priority first [OK]

Common Mistakes:

• Assuming PriorityQueue dequeues highest priority first by default
• Trying to sort queue manually
• Using Stack which doesn't handle priorities