Stack Concept and LIFO Principle in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an empty stack of integers.

DSA C#

Stack<int> stack = new Stack<int>[1];

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

A[]

B()

C{}

D<>

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets [] instead of parentheses.

Using curly braces {} which are for collection initializers.

Using angle brackets <> which define generic types.

In C#, to create a new instance of a Stack, you use parentheses ().

2fill in blank

medium

Complete the code to add the number 10 to the stack.

DSA C#

stack.[1](10);

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

APush

BEnqueue

CInsert

DAdd

Attempts:

3 left

💡 Hint

Common Mistakes

Using Add which is for lists, not stacks.

Using Insert which is for lists at specific positions.

Using Enqueue which is for queues, not stacks.

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

3fill in blank

hard

Fix the error in the code to remove the top item from the stack.

DSA C#

int top = stack.[1]();

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

ADequeue

BRemove

CDelete

DPop

Attempts:

3 left

💡 Hint

Common Mistakes

Using Remove or Delete which are not stack methods.

Using Dequeue which is for queues.

Pop removes and returns the top item from the stack following LIFO.

4fill in blank

hard

Fill both blanks to check if the stack is empty and print a message.

DSA C#

if (stack.[1] == [2]) {
    Console.WriteLine("Stack is empty");
}

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

ACount

B0

CLength

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using Length which is not a Stack property.

Comparing to null which is not valid for Count.

Stack.Count gives the number of items; if it equals 0, the stack is empty.

5fill in blank

hard

Fill all three blanks to create a stack, add 5, and then remove the top item.

DSA C#

Stack<int> stack = new Stack<int>[1];
stack.[2](5);
int removed = stack.[3]();

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

A()

BPush

CPop

DAdd

Attempts:

3 left

💡 Hint

Common Mistakes

Using Add instead of Push to add items.

Using Add or Dequeue instead of Pop to remove items.

Create stack with (), add item with Push, remove top with Pop.

Practice

1. What does the LIFO principle mean in the context of a stack?

easy

A. Last In, First Out - the first element added is the first to be removed

B. First In, First Out - the first element added is the first to be removed

C. Last In, First Out - the last element added is the first to be removed

D. First In, Last Out - the first element added is the last to be removed

Solution

1. Step 1: Understand LIFO meaning

   LIFO means the last item you put in is the first one you take out.

2. Step 2: Apply to stack behavior

   Stacks follow LIFO, so the last pushed element is popped first.

3. Final Answer:

   Last In, First Out - the last element added is the first to be removed -> Option C

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember stack is like a stack of plates, last added removed first [OK]

Common Mistakes:

• Confusing LIFO with FIFO
• Thinking first added is removed first
• Mixing up the order of insertion and removal

2. Which of the following is the correct way to push an integer 10 onto a stack named myStack in C#?

easy

A. myStack.Enqueue(10);

B. myStack.Push(10);

C. myStack.Insert(10);

D. myStack.Add(10);

Solution

1. Step 1: Recall stack push method

   In C#, the method to add an element to a stack is Push.

2. Step 2: Check method names

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

3. Final Answer:

   myStack.Push(10); -> Option B

4. Quick Check:

   Push() adds element to stack [OK]

Hint: Use Push() to add elements to stack in C# [OK]

Common Mistakes:

• Using Add() or Insert() which are not stack methods
• Confusing stack with queue methods like Enqueue()
• Forgetting parentheses in method call

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

Stack<int> stack = new Stack<int>();
stack.Push(5);
stack.Push(10);
stack.Push(15);
Console.WriteLine(stack.Pop());
Console.WriteLine(stack.Peek());

medium

A. 15
10

B. 5
10

C. 10
15

D. 15
15

Solution

1. Step 1: Trace Push operations

   Elements pushed in order: 5 (bottom), 10, 15 (top).

2. Step 2: Trace Pop and Peek

   Pop removes and returns 15 (top). Peek returns new top 10 without removing.

3. Final Answer:

   15\n10 -> Option A

4. Quick Check:

   Pop() = 15, Peek() = 10 [OK]

Hint: Pop returns top and removes it; Peek returns top without removing [OK]

Common Mistakes:

• Mixing Pop and Peek outputs
• Assuming Pop does not remove element
• Confusing order of pushed elements

4. Identify the error in the following C# code snippet using a stack:

Stack<string> stack = new Stack<string>();
stack.Push("apple");
stack.Push("banana");
string fruit = stack.Pop();
string topFruit = stack.Top();

medium

A. Stack must be initialized with capacity

B. Pop() cannot be assigned to a string variable

C. Push() method requires two arguments

D. Stack does not have a method named Top()

Solution

1. Step 1: Check stack methods

   Stack in C# has Peek() to view top element, not Top().

2. Step 2: Validate other statements

   Pop() returns string, Push() takes one argument, no capacity needed.

3. Final Answer:

   Stack does not have a method named Top() -> Option D

4. Quick Check:

   Use Peek() to view top element [OK]

Hint: Use Peek() to see top element, not Top() [OK]

Common Mistakes:

• Using Top() instead of Peek()
• Thinking Pop() returns void
• Assuming Push() needs capacity argument

5. Given a stack with elements pushed in this order: 1, 2, 3, 4, 5. You pop two elements, then push 6 and 7. What is the order of elements if you pop all remaining elements one by one?

hard

A. 7, 6, 3, 2, 1

B. 3, 2, 6, 7

C. 5, 4, 6, 7

D. 7, 6, 3, 2

Solution

1. Step 1: Track initial pushes

   Stack after pushes: bottom 1,2,3,4,5 top.

2. Step 2: Pop two elements

   Pop removes 5 then 4; stack now 1,2,3 (bottom to top).

3. Step 3: Push 6 and 7

   Stack now: 1,2,3,6,7 (bottom to top).

4. Step 4: Pop all remaining elements

   Pop order (first popped to last): 7,6,3,2,1.

5. Final Answer:

   7, 6, 3, 2, 1 -> Option A

6. Quick Check:

   Pop order after operations = 7,6,3,2,1 [OK]

Hint: Pop removes last pushed; track pushes and pops stepwise [OK]

Common Mistakes:

• Forgetting to remove popped elements
• Mixing push and pop order
• Ignoring elements still in stack after operations