Stack Concept and LIFO Principle in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this stack operation sequence?

Consider a stack where we push integers 1, 2, 3 in order, then pop one element, then push 4. What is the printed stack from top to bottom?

DSA C#

Stack<int> stack = new Stack<int>();
stack.Push(1);
stack.Push(2);
stack.Push(3);
stack.Pop();
stack.Push(4);

while(stack.Count > 0)
{
    Console.Write(stack.Pop());
    if(stack.Count > 0) Console.Write(" -> ");
}
Console.WriteLine(" -> null");

A 4 -> 2 -> 1 -> null

B 2 -> 4 -> 1 -> null

C 1 -> 2 -> 4 -> null

D 3 -> 2 -> 1 -> null

💡 Hint

Remember that stack follows Last In First Out (LIFO). The last pushed element is popped first.

We push 1, 2, 3. Stack top is 3. Then pop removes 3. Stack top is 2. Then push 4. Stack top is 4. So popping all prints 4, 2, 1.

🧠 Conceptual

intermediate

Which statement best describes the LIFO principle in stacks?

Choose the statement that correctly explains the Last In First Out (LIFO) principle used by stacks.

A The first element added is the first one to be removed.

B Elements are removed in random order regardless of insertion.

C The last element added is the first one to be removed.

D Elements are removed in the same order they were added.

💡 Hint

Think about which element is on top of the stack.

LIFO means the last element pushed onto the stack is the first one popped off.

🔧 Debug

advanced

What error occurs when popping from an empty stack?

Given this code snippet, what error will be raised when the Pop method is called on an empty stack?

DSA C#

Stack<int> stack = new Stack<int>();
stack.Pop();

A InvalidOperationException

B No error, returns default value

C IndexOutOfRangeException

D NullReferenceException

💡 Hint

Check what exception Stack.Pop throws if empty.

Calling Pop on an empty stack throws InvalidOperationException in C#.

🚀 Application

advanced

What is the stack content after this sequence of operations?

Start with an empty stack. Perform these operations in order: Push(10), Push(20), Pop(), Push(30), Push(40), Pop(), Push(50). What is the stack content from top to bottom?

A 50 -> 40 -> 10 -> null

B 50 -> 30 -> 10 -> null

C 40 -> 30 -> 10 -> null

D 30 -> 50 -> 10 -> null

💡 Hint

Track each push and pop carefully, remembering LIFO.

Push 10, stack: 10
Push 20, stack: 20,10
Pop removes 20, stack: 10
Push 30, stack: 30,10
Push 40, stack: 40,30,10
Pop removes 40, stack: 30,10
Push 50, stack: 50,30,10

❓ Predict Output

expert

What is the output of this C# stack iteration code?

Given this code, what is printed to the console?

DSA C#

Stack<string> stack = new Stack<string>();
stack.Push("apple");
stack.Push("banana");
stack.Push("cherry");

foreach(var item in stack)
{
    Console.Write(item + " -> ");
}
Console.WriteLine("null");

A apple -> cherry -> banana -> null

B apple -> banana -> cherry -> null

C banana -> cherry -> apple -> null

D cherry -> banana -> apple -> null

💡 Hint

Stack enumeration goes from top to bottom without removing elements.

Stack enumerates from top to bottom, so order is cherry, banana, apple.

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