Queue and Stack behavior in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main difference between a Queue and a Stack?

A Queue follows First-In-First-Out (FIFO) order, meaning the first item added is the first removed. A Stack follows Last-In-First-Out (LIFO) order, meaning the last item added is the first removed.

beginner

In C#, which method adds an item to a Queue?

The Enqueue() method adds an item to the end of the Queue.

beginner

How do you remove the top item from a Stack in C#?

Use the Pop() method to remove and return the item at the top of the Stack.

intermediate

What happens if you call Dequeue() on an empty Queue in C#?

It throws an InvalidOperationException because there are no items to remove.

intermediate

Explain the behavior of Peek() method in both Queue and Stack.

The Peek() method returns the next item to be removed without removing it. For a Queue, it returns the item at the front. For a Stack, it returns the item at the top.

Which method adds an element to the top of a Stack in C#?

A Pop()

B Enqueue()

C Push()

D Dequeue()

The Push() method adds an element to the top of the Stack.

What order does a Queue follow?

A First-In-First-Out (FIFO)

B Last-In-First-Out (LIFO)

C Random order

D Sorted order

A Queue follows First-In-First-Out (FIFO) order.

What does the Pop() method do in a Stack?

A Removes and returns the top item

B Returns the bottom item without removing

C Adds an item to the bottom

D Removes the first item added

Pop() removes and returns the top item of the Stack.

Which method removes the front item from a Queue?

A Pop()

B Push()

C Peek()

D Dequeue()

Dequeue() removes and returns the front item of the Queue.

What will happen if you call Peek() on an empty Stack?

A Returns null

B Throws InvalidOperationException

C Returns default value

D Removes an item

Calling Peek() on an empty Stack throws InvalidOperationException.

Describe how Queue and Stack differ in the order they remove items.

Think about a line of people versus a stack of plates.

Explain the purpose of the Enqueue, Dequeue, Push, and Pop methods in C# collections.

Match methods to adding or removing items in Queue or Stack.

Practice

1. Which data structure removes elements in the order they were added, like a line at a grocery store?

easy

A. Array

B. Stack

C. Dictionary

D. Queue

Solution

1. Step 1: Understand FIFO behavior

   A queue removes elements in the order they were added, called First-In-First-Out (FIFO).

2. Step 2: Match behavior to real-life example

   A line at a grocery store is FIFO, so the queue matches this behavior.

3. Final Answer:

   Queue -> Option D

4. Quick Check:

   FIFO = Queue [OK]

Hint: FIFO means first in, first out like a queue line [OK]

Common Mistakes:

• Confusing stack with queue
• Thinking stack is FIFO
• Mixing array behavior with queue
• Assuming dictionary has order

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

easy

A. stack.Push(item);

B. stack.Enqueue(item);

C. stack.Add(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 A

4. Quick Check:

   Push adds to Stack [OK]

Hint: Use Push() to add to Stack, Enqueue() for Queue [OK]

Common Mistakes:

• Using Enqueue() on Stack
• Using Add() or Insert() which don't exist
• Confusing Stack and Queue methods
• Syntax errors with method calls

3. What is the output of this C# code?

var stack = new Stack<int>();
stack.Push(1);
stack.Push(2);
stack.Push(3);
Console.WriteLine(stack.Pop());
Console.WriteLine(stack.Peek());

medium

A. 3\n2

B. 1\n2

C. 2\n3

D. 3\n3

Solution

1. Step 1: Trace Push operations

   Stack after pushes: bottom=1, middle=2, top=3.

2. Step 2: Execute Pop and Peek

   Pop() removes and returns top (3). Peek() returns new top (2) without removing.

3. Final Answer:

   3\n2 -> Option A

4. Quick Check:

   Pop=3, Peek=2 [OK]

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

Common Mistakes:

• Mixing Pop and Peek results
• Assuming FIFO order
• Confusing stack order
• Forgetting Pop removes item

4. Identify the error in this C# code using Queue:

var queue = new Queue<string>();
queue.Push("apple");
queue.Enqueue("banana");
Console.WriteLine(queue.Dequeue());

medium

A. Dequeue() returns last item added

B. Queue does not have Push() method

C. Enqueue() should be Dequeue()

D. Queue cannot store strings

Solution

1. Step 1: Check Queue methods

   Queue uses Enqueue() to add, not Push().

2. Step 2: Identify incorrect method usage

   Calling Push() on Queue causes a compile error.

3. Final Answer:

   Queue does not have Push() method -> Option B

4. Quick Check:

   Queue uses Enqueue, no Push [OK]

Hint: Queue uses Enqueue(), Stack uses Push() [OK]

Common Mistakes:

• Using Push() on Queue
• Confusing Enqueue and Dequeue
• Thinking Dequeue returns last item
• Assuming Queue can't hold strings

5. You want to reverse the order of words in a sentence using C#. Which data structure is best and why?

string sentence = "hello world from C#";

hard

A. Queue, because it keeps original order using FIFO

B. Dictionary, because it stores key-value pairs

C. Stack, because it reverses order using LIFO

D. List, because it sorts items automatically

Solution

1. Step 1: Understand the goal

   Reversing words means last word should come first, so order is reversed.

2. Step 2: Choose data structure behavior

   Stack uses Last-In-First-Out (LIFO), perfect for reversing order.

3. Step 3: Eliminate other options

   Queue keeps order (FIFO), Dictionary stores pairs unordered, List does not reverse automatically.

4. Final Answer:

   Stack, because it reverses order using LIFO -> Option C

5. Quick Check:

   Reverse order = Stack (LIFO) [OK]

Hint: Use Stack to reverse order with LIFO behavior [OK]

Common Mistakes:

• Choosing Queue for reversing
• Thinking List auto-sorts
• Using Dictionary for order
• Ignoring LIFO vs FIFO difference