Pop Operation on Stack in DSA C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the pop operation do in a stack?

The pop operation removes the top element from the stack and returns it. It reduces the stack size by one.

beginner

What happens if you try to pop from an empty stack?

Popping from an empty stack causes an underflow error because there is no element to remove.

intermediate

In C++, which standard container can be used to implement a stack with pop operation?

The std::stack container adapter can be used. It provides pop() to remove the top element.

beginner

What is the time complexity of the pop operation on a stack?

The pop operation runs in O(1) time because it only removes the top element without traversing the stack.

beginner

Explain the difference between pop and peek (or top) operations on a stack.

Pop removes and returns the top element, changing the stack size. Peek (or top) returns the top element without removing it.

What does the pop operation do in a stack?

A Removes the top element

B Adds an element to the bottom

C Returns the size of the stack

D Clears all elements

Pop removes the top element from the stack.

What error occurs if you pop from an empty stack?

A Overflow

B Underflow

C Segmentation fault

D Syntax error

Underflow happens when popping from an empty stack.

What is the time complexity of the pop operation on a stack?

A O(1)

B O(log n)

C O(n)

D O(n^2)

Pop runs in constant time O(1).

Which C++ container provides a pop() method for stacks?

A std::list

B std::queue

C std::vector

D std::stack

std::stack provides pop() to remove the top element.

What is the difference between pop and peek operations?

A Pop adds element; peek removes element

B Pop returns size; peek clears stack

C Pop removes top element; peek returns it without removing

D Pop and peek do the same

Pop removes and returns the top element; peek only returns it.

Describe the pop operation on a stack and what happens when the stack is empty.

Think about what happens to the top element and the stack size.

Explain the difference between pop and peek operations in a stack.

One changes the stack, the other does not.

Practice

1. What does the pop() operation do in a stack?

easy

A. Adds a new element to the bottom of the stack

B. Removes the top element from the stack

C. Returns the size of the stack

D. Clears all elements from the stack

Solution

1. Step 1: Understand stack behavior

   A stack follows Last In First Out (LIFO) principle, so the last added element is removed first.

2. Step 2: Define pop operation

   The pop operation removes the top element from the stack, which is the last inserted item.

3. Final Answer:

   Removes the top element from the stack -> Option B

4. Quick Check:

   pop() removes top element [OK]

Hint: Pop always removes the last added item [OK]

Common Mistakes:

• Thinking pop adds elements
• Confusing pop with peek or size
• Assuming pop clears the stack

2. Which of the following is the correct syntax to pop an element from a stack named stk in C++ STL?

easy

A. stk.pop();

B. pop(stk);

C. stk.pop_element();

D. stk.remove();

Solution

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

   The standard stack class in C++ uses member functions like push() and pop() called on the stack object.

2. Step 2: Identify correct pop syntax

   The correct way to remove the top element is calling pop() as stk.pop();.

3. Final Answer:

   stk.pop(); -> Option A

4. Quick Check:

   Use object.pop() to remove top [OK]

Hint: Use object.pop() to remove top element [OK]

Common Mistakes:

• Using pop() as a free function
• Using incorrect method names like pop_element()
• Trying to remove with remove() which doesn't exist

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

std::stack<int> stk;
stk.push(10);
stk.push(20);
stk.pop();
std::cout << stk.top();

medium

A. 0

B. 20

C. 10

D. Compilation error

Solution

1. Step 1: Trace push operations

   First, 10 is pushed, then 20 is pushed on top. Stack top is now 20.

2. Step 2: Apply pop and check top

   pop() removes the top element (20), so top becomes 10. Printing top outputs 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   After pop, top is 10 [OK]

Hint: Pop removes last pushed, top shows new top [OK]

Common Mistakes:

• Assuming top remains 20 after pop
• Confusing pop with peek
• Expecting 0 or error output

4. Identify the error in the following code snippet that uses a stack:

std::stack<int> stk;
stk.pop();
stk.push(5);

medium

A. No error, code runs fine

B. Push should be called before pop

C. Stack declaration is incorrect

D. Pop called on empty stack causes runtime error

Solution

1. Step 1: Analyze stack state before pop

   The stack is empty initially. Calling pop() on empty stack is invalid and causes runtime error.

2. Step 2: Check order of operations

   Pop is called before any push, so no element exists to remove, causing error.

3. Final Answer:

   Pop called on empty stack causes runtime error -> Option D

4. Quick Check:

   Pop on empty stack causes error [OK]

Hint: Never pop from empty stack [OK]

Common Mistakes:

• Ignoring empty stack before pop
• Thinking push order fixes error
• Assuming pop on empty is safe

5. Given a stack with elements 1, 2, 3, 4 pushed in that order (1 at bottom, 4 at top), what will be the stack content after performing two pop() operations?

hard

A. 1 -> 2 -> null

B. 3 -> 4 -> null

C. 1 -> 2 -> 3 -> null

D. 4 -> 3 -> 2 -> 1 -> null

Solution

1. Step 1: Understand initial stack order

   Elements pushed in order: 1 (bottom), 2, 3, 4 (top). Stack looks like 1 -> 2 -> 3 -> 4 -> null.

2. Step 2: Apply two pop operations

   First pop removes 4 (top), second pop removes 3 (new top). Remaining stack is 1 -> 2 -> null.

3. Final Answer:

   1 -> 2 -> null -> Option A

4. Quick Check:

   Two pops remove top two elements [OK]

Hint: Pop removes top elements one by one [OK]

Common Mistakes:

• Confusing order of elements after pop
• Listing popped elements instead of remaining
• Reversing stack order incorrectly