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 initially empty. We perform these operations in order:
push(10), push(20), pop(), push(30), pop(), push(40), push(50), pop()
What is the content of the stack from top to bottom after these operations?

DSA C++

std::stack<int> s;
s.push(10);
s.push(20);
s.pop();
s.push(30);
s.pop();
s.push(40);
s.push(50);
s.pop();
// Print stack from top to bottom

A 20 -> 10 -> null

B 50 -> 40 -> 10 -> null

C 40 -> 10 -> null

D 30 -> 10 -> null

💡 Hint

Remember pop removes the top element. Trace each step carefully.

After the operations, the stack has 40 on top and 10 below. The last pop removed 50, so 40 is top.

🧠 Conceptual

intermediate

Which statement correctly describes the LIFO principle in stacks?

Choose the statement that best explains how the Last In First Out (LIFO) principle works in a stack.

A Elements are removed in the reverse order they were added.

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

C Elements are removed randomly regardless of insertion order.

D The last element added is never removed.

💡 Hint

Think about which element is on top and removed first.

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

🔧 Debug

advanced

What error does this stack code produce?

Given the code below, what error will occur when running it?

std::stack s;
s.push(1);
s.pop();
s.pop();

DSA C++

std::stack<int> s;
s.push(1);
s.pop();
s.pop();

A Logic error: stack size incorrect

B Runtime error: popping from empty stack

C Compile error: pop() not defined

D No error, runs fine

💡 Hint

Check what happens if you pop more times than you push.

Popping from an empty stack causes a runtime error (undefined behavior).

🚀 Application

advanced

How many elements remain after this sequence?

A stack starts empty. We perform:
push(5), push(10), push(15), pop(), push(20), pop(), pop()
How many elements are left in the stack?

A 0

B 2

C 3

D 1

💡 Hint

Count pushes and pops carefully.

After all operations, only one element remains in the stack.

❓ Predict Output

expert

What is printed by this C++ stack code?

What is the output of this code?

#include <iostream>
#include <stack>

int main() {
    std::stack s;
    for (int i = 1; i <= 3; ++i) s.push(i);
    int sum = 0;
    while (!s.empty()) {
        sum += s.top();
        s.pop();
    }
    std::cout << sum << std::endl;
    return 0;
}

DSA C++

#include <iostream>
#include <stack>

int main() {
    std::stack<int> s;
    for (int i = 1; i <= 3; ++i) s.push(i);
    int sum = 0;
    while (!s.empty()) {
        sum += s.top();
        s.pop();
    }
    std::cout << sum << std::endl;
    return 0;
}

A 6

B 3

C 0

D 9

💡 Hint

Sum all elements popped from the stack.

Stack contains 1,2,3 pushed. Sum is 1+2+3=6.

Practice

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

easy

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

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

C. Elements are removed randomly

D. Elements are removed based on their value

Solution

1. Step 1: Understand LIFO principle

   LIFO means the last element added to the stack is the first one to be removed.

2. Step 2: Match with 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 removed first -> Option A

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember stack removes newest item first [OK]

Common Mistakes:

• Confusing LIFO with FIFO
• Thinking stack removes oldest element first
• Assuming random removal order

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

easy

A. s.add(10);

B. s.insert(10);

C. push(s, 10);

D. s.push(10);

Solution

1. Step 1: Recall C++ STL stack push syntax

   In C++ STL, the method to add an element to stack is push() called on the stack object.

2. Step 2: Verify correct method usage

   Using s.push(10); correctly pushes 10 onto stack s.

3. Final Answer:

   s.push(10); -> Option D

4. Quick Check:

   Stack push method = push() [OK]

Hint: Use s.push(value) to add in C++ STL stack [OK]

Common Mistakes:

• Using non-existent methods like add or insert
• Calling push as a free function
• Confusing vector methods with stack

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

std::stack<int> s;
s.push(5);
s.push(10);
s.push(15);
int x = s.top();
s.pop();
int y = s.top();
std::cout << x << " " << y << std::endl;

medium

A. 15 10

B. 5 10

C. 10 5

D. 15 5

Solution

1. Step 1: Trace stack push operations

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

2. Step 2: Check top and pop operations

   First, x = s.top() gives 15. Then s.pop() removes 15. Next, y = s.top() gives 10.

3. Final Answer:

   15 10 -> Option A

4. Quick Check:

   Top after push = 15, after pop = 10 [OK]

Hint: Top shows last pushed; pop removes it [OK]

Common Mistakes:

• Confusing order of elements after pop
• Assuming pop returns value
• Mixing bottom and top elements

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

std::stack<int> s;
s.pop();
s.push(20);

medium

A. Syntax error in stack declaration

B. Cannot pop from an empty stack; causes runtime error

C. push should come before pop always

D. No error; code runs fine

Solution

1. Step 1: Analyze stack operations order

   The code calls pop() on an empty stack before pushing any element.

2. Step 2: Understand pop behavior on empty stack

   Calling pop() on empty stack is undefined and causes runtime error.

3. Final Answer:

   Cannot pop from an empty stack; causes runtime error -> Option B

4. Quick Check:

   Pop empty stack = runtime error [OK]

Hint: Never pop before pushing any element [OK]

Common Mistakes:

• Ignoring empty stack before pop
• Assuming pop returns value
• Thinking push order fixes pop error

5. Given a stack with elements pushed in this order: 3, 7, 9, 2, 5. Which sequence of pop() operations will produce the output 5 2 9 when printing popped elements?

hard

A. Pop twice, push 1, then pop once

B. Pop once, push 4, then pop twice

C. Pop three times consecutively

D. Pop once, pop again, then push 6 and pop

Solution

1. Step 1: Understand stack order after pushes

   Stack top to bottom: 5 (top), 2, 9, 7, 3 (bottom).

2. Step 2: Pop three times consecutively

   First pop: 5, second pop: 2, third pop: 9. This matches output sequence.

3. Final Answer:

   Pop three times consecutively -> Option C

4. Quick Check:

   Pop order matches output 5 2 9 [OK]

Hint: Pop in order to get last pushed elements first [OK]

Common Mistakes:

• Adding extra pushes changes output
• Popping fewer times than needed
• Confusing push/pop order