Queue vs Stack When to Use Which in DSA C# - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an item to the end of a queue.

DSA C#

Queue<int> numbers = new Queue<int>();
numbers.[1](5);

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

APush

BEnqueue

CAdd

DInsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using Push which is for stacks, not queues.

Using Add which is not a method of Queue.

In C#, Enqueue adds an item to the end of a queue.

2fill in blank

medium

Complete the code to remove the last item added to a stack.

DSA C#

Stack<string> books = new Stack<string>();
string lastBook = books.[1]();

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

APop

BRemove

CDequeue

DPeek

Attempts:

3 left

💡 Hint

Common Mistakes

Using Dequeue which is for queues, not stacks.

Using Peek which only views the top item without removing it.

Pop removes and returns the last item added to a stack.

3fill in blank

hard

Fix the error in the code to check the next item to be removed from a queue without removing it.

DSA C#

Queue<char> letters = new Queue<char>();
char nextLetter = letters.[1]();

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

APop

BTop

CPeek

DDequeue

Attempts:

3 left

💡 Hint

Common Mistakes

Using Pop which removes from a stack, not a queue.

Using Dequeue which removes the item.

Peek returns the next item in the queue without removing it.

4fill in blank

hard

Fill both blanks to create a stack and add an item to it.

DSA C#

Stack<int> numbers = new [1]<int>();
numbers.[2](10);

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

AStack

BQueue

CPush

DEnqueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using Queue instead of Stack.

Using Enqueue which is for queues.

Use Stack to create a stack and Push to add an item.

5fill in blank

hard

Fill all three blanks to create a queue, add an item, and remove the first item.

DSA C#

Queue<string> tasks = new [1]<string>();
tasks.[2]("Email");
string firstTask = tasks.[3]();

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

AStack

BEnqueue

CDequeue

DQueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using Stack instead of Queue.

Using Push or Pop which are for stacks.

Create a Queue, add with Enqueue, and remove with Dequeue.

Practice

1. Which data structure should you use when you want to process items in the order they arrive, like customers in a line?

easy

A. Queue

B. Stack

C. Array

D. Dictionary

Solution

1. Step 1: Understand the order of processing

   Customers in a line are served in the order they arrive, which is First In First Out (FIFO).

2. Step 2: Match the data structure behavior

   A Queue follows FIFO, so it fits this scenario perfectly.

3. Final Answer:

   Queue -> Option A

4. Quick Check:

   FIFO = Queue [OK]

Hint: FIFO means use Queue, like a line of customers [OK]

Common Mistakes:

• Choosing Stack which is LIFO, not FIFO
• Confusing Array with order behavior
• Using Dictionary which is unordered

2. Which of the following is the correct way to add an item to a Stack in C#?

easy

A. stack.Enqueue(item);

B. stack.Add(item);

C. stack.Push(item);

D. stack.Insert(item);

Solution

1. Step 1: Recall Stack method names in C#

   Stack uses Push() to add items on top.

2. Step 2: Identify correct method

   Enqueue is for Queue, Add and Insert are not Stack methods.

3. Final Answer:

   stack.Push(item); -> Option C

4. Quick Check:

   Push adds to Stack [OK]

Hint: Push adds to Stack; Enqueue adds to Queue [OK]

Common Mistakes:

• Using Enqueue with Stack
• Using Add or Insert which are not Stack methods
• Confusing method names between Stack and Queue

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

var stack = new Stack<int>();
stack.Push(10);
stack.Push(20);
stack.Push(30);
Console.WriteLine(stack.Pop());
Console.WriteLine(stack.Peek());

medium

A. 10\n20

B. 30\n20

C. 30\n30

D. 20\n10

Solution

1. Step 1: Trace Push operations on Stack

   Push 10, then 20, then 30. Stack top is 30.

2. Step 2: Execute Pop and Peek

   Pop removes and returns 30. Peek returns current top which is now 20.

3. Final Answer:

   30\n20 -> Option B

4. Quick Check:

   Pop = 30, Peek = 20 [OK]

Hint: Pop removes top, Peek shows top without removing [OK]

Common Mistakes:

• Confusing Pop and Peek results
• Assuming Stack is FIFO
• Mixing up order of Push calls

4. The following code is intended to process tasks in the order they arrive. What is the error?

var stack = new Stack<string>();
stack.Push("Task1");
stack.Push("Task2");
stack.Push("Task3");
while(stack.Count > 0)
{
    Console.WriteLine(stack.Pop());
}

medium

A. Stack processes tasks in reverse order, not arrival order

B. Stack does not have Push method

C. Missing initialization of Stack

D. Pop method does not remove items

Solution

1. Step 1: Understand Stack order behavior

   Stack is Last In First Out (LIFO), so tasks are processed in reverse order.

2. Step 2: Identify mismatch with requirement

   Processing tasks in arrival order requires FIFO, so Stack is wrong choice here.

3. Final Answer:

   Stack processes tasks in reverse order, not arrival order -> Option A

4. Quick Check:

   Stack = LIFO, needs FIFO for arrival order [OK]

Hint: Stack reverses order; use Queue for arrival order [OK]

Common Mistakes:

• Thinking Stack processes FIFO
• Ignoring order requirement
• Assuming Pop does not remove items

5. You need to design a system that tracks browser history allowing users to go back to the previous page. Which data structure is best and why?

hard

A. Array, because it stores pages sequentially

B. Queue, because it processes pages in order visited

C. Dictionary, because it stores pages with keys

D. Stack, because it allows going back to the last visited page first

Solution

1. Step 1: Understand browser back behavior

   Going back means visiting the last page visited first, which is Last In First Out (LIFO).

2. Step 2: Match data structure to behavior

   Stack follows LIFO, so it fits the browser history back feature perfectly.

3. Final Answer:

   Stack, because it allows going back to the last visited page first -> Option D

4. Quick Check:

   LIFO = Stack for browser back [OK]

Hint: Back button = LIFO = Stack [OK]

Common Mistakes:

• Choosing Queue which is FIFO
• Using Dictionary which is unordered
• Assuming Array handles back navigation automatically