Why Stack Exists and What Problems It Solves in DSA C# - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an item onto the stack.

DSA C#

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

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

AInsert

BAdd

CAppend

DPush

Attempts:

3 left

💡 Hint

Common Mistakes

Using Add or Append which are not stack methods.

Trying to use Insert which is for lists.

The Push method adds an item to the top of the stack.

2fill in blank

medium

Complete the code to remove the top item from the stack.

DSA C#

Stack<string> stack = new Stack<string>();
stack.Push("apple");
string top = stack.[1]();

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

ARemove

BDelete

CPop

DTake

Attempts:

3 left

💡 Hint

Common Mistakes

Using Remove or Delete which are not stack methods.

Trying to use Take which is not a stack method.

The Pop method removes and returns the top item from the stack.

3fill in blank

hard

Fix the error in the code to check the top item without removing it.

DSA C#

Stack<char> stack = new Stack<char>();
stack.Push('A');
char top = stack.[1]();

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

APeek

BTop

CPop

DFront

Attempts:

3 left

💡 Hint

Common Mistakes

Using Pop which removes the item.

Using Top or Front which are not stack methods.

The Peek method returns the top item without removing it.

4fill in blank

hard

Fill both blanks to create a stack and check if it is empty.

DSA C#

Stack<double> stack = new [1]<double>();
bool isEmpty = stack.[2] == 0;

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

AStack

BCount

CLength

DList

Attempts:

3 left

💡 Hint

Common Mistakes

Using List instead of Stack.

Using Length which is not a property of Stack.

Create a stack with Stack and check if Count is zero to know if it's empty.

5fill in blank

hard

Fill all three blanks to push items and then pop the top item.

DSA C#

Stack<string> stack = new [1]<string>();
stack.[2]("first");
stack.[2]("second");
string top = stack.[3]();

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

AStack

BPush

CPop

DAdd

Attempts:

3 left

💡 Hint

Common Mistakes

Using Add instead of Push.

Using Push to remove items.

Create a stack with Stack, add items with Push, and remove the top with Pop.

Practice

1. Why do we use a stack data structure in programming?

easy

A. To keep track of function calls and return in the correct order

B. To store data in random order for quick access

C. To sort numbers in ascending order automatically

D. To store large amounts of data permanently

Solution

1. Step 1: Understand the purpose of a stack

   A stack stores items in a last-in, first-out order, which matches how function calls return in programming.

2. Step 2: Match stack use to function call behavior

   When functions call other functions, the last called function must finish first, so a stack helps keep track of this order.

3. Final Answer:

   To keep track of function calls and return in the correct order -> Option A

4. Quick Check:

   Stack = function call tracking [OK]

Hint: Stack follows last-in, first-out like undo actions [OK]

Common Mistakes:

• Thinking stack stores data randomly
• Confusing stack with queue
• Assuming stack sorts data automatically

2. Which of the following is the correct way to push an item onto a stack in C#?

easy

A. stack.Add(item);

B. stack.Insert(item);

C. stack.Push(item);

D. stack.Enqueue(item);

Solution

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

   The Stack class in C# uses Push() to add an item on top.

2. Step 2: Identify incorrect methods

   Add() and Insert() are not stack methods; Enqueue() is for queues.

3. Final Answer:

   stack.Push(item); -> Option C

4. Quick Check:

   Push adds item to stack [OK]

Hint: Use Push() to add, Pop() to remove from stack [OK]

Common Mistakes:

• Using Add() which is for lists
• Using Enqueue() which is for queues
• Confusing Insert() with stack methods

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

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

medium

A. 30

B. 20

C. Error

D. 10

Solution

1. Step 1: Trace the stack operations

   Items pushed in order: 10, then 20, then 30. Stack top is 30.

2. Step 2: Pop removes and returns top item

   Pop() removes 30 and returns it, so top = 30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Last pushed item popped = 30 [OK]

Hint: Pop() returns last pushed item [OK]

Common Mistakes:

• Thinking Pop() returns first pushed item
• Confusing Push() and Pop() order
• Expecting error due to missing code

4. The following code tries to pop an item from an empty stack. What is the problem?

Stack<string> stack = new Stack<string>();
string item = stack.Pop();
Console.WriteLine(item);

medium

A. It will print null

B. It will throw an InvalidOperationException

C. It will print an empty string

D. It will compile error

Solution

1. Step 1: Understand Pop() on empty stack behavior

   Pop() on an empty stack throws InvalidOperationException in C#.

2. Step 2: Check code behavior

   Since stack is empty, calling Pop() causes runtime exception, not printing anything.

3. Final Answer:

   It will throw an InvalidOperationException -> Option B

4. Quick Check:

   Pop empty stack = exception [OK]

Hint: Pop() on empty stack throws exception [OK]

Common Mistakes:

• Expecting null or empty string output
• Thinking code won't compile
• Ignoring runtime exceptions

5. You want to reverse a string using a stack in C#. Which approach correctly uses a stack to reverse the string "hello"?

hard

A. Use stack to sort characters alphabetically

B. Push all characters, then push them again to reverse

C. Pop characters first, then push to reverse

D. Push each character, then pop all to build reversed string

Solution

1. Step 1: Understand stack reversal logic

   Pushing characters in order and popping them reverses the order because stack is last-in, first-out.

2. Step 2: Identify correct approach

   Push each character of "hello" then pop all to get "olleh". Other options do not reverse correctly.

3. Final Answer:

   Push each character, then pop all to build reversed string -> Option D

4. Quick Check:

   Stack reversal = push then pop [OK]

Hint: Push then pop characters to reverse string [OK]

Common Mistakes:

• Pushing twice without popping
• Popping before pushing
• Using stack to sort instead of reverse