Stack operations (push, pop, peek) in Data Structures Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the Peek Operation

What does the peek operation do in a stack?

A Removes the bottom element of the stack

B Removes and returns the top element of the stack

C Returns the top element without removing it

D Returns the size of the stack

💡 Hint

Think about whether the element stays in the stack after the operation.

The peek operation returns the top element of the stack without removing it, allowing you to see what is on top without changing the stack.

📋 Factual

intermediate

Stack Behavior After Push and Pop

Consider an empty stack. After pushing elements 5, 10, and 15 in that order, then popping once, what is the top element?

A 5

B 10

C 15

D Stack is empty

💡 Hint

Remember that pop removes the top element.

After pushing 5, 10, 15, the stack top is 15. Popping removes 15, so the new top is 10.

🔍 Analysis

advanced

Stack Underflow Scenario

What happens if you perform a pop operation on an empty stack?

A Causes an error or exception called stack underflow

B Removes the bottom element instead

C Returns null or equivalent empty value

D Adds a default element to the stack

💡 Hint

Think about what it means to remove an element when none exist.

Trying to pop from an empty stack causes a stack underflow error because there is no element to remove.

❓ Comparison

advanced

Difference Between Push and Pop

Which statement correctly describes the difference between push and pop operations on a stack?

A Push adds a new element on top; pop removes the top element

B Both push and pop remove elements but from different ends

C Push removes the top element; pop adds a new element on top

D Push and pop both add elements but at different positions

💡 Hint

Think about how elements enter and leave the stack.

Push adds an element to the top of the stack, while pop removes the top element.

❓ Reasoning

expert

Stack State After Multiple Operations

Starting with an empty stack, the following operations are performed in order: push(3), push(7), pop(), push(9), peek(). What is the value returned by the peek operation?

A 3

B 7

C Stack is empty

D 9

💡 Hint

Track each operation step-by-step to see what remains on the stack.

After push(3) and push(7), stack top is 7. Pop removes 7, leaving 3. Push(9) adds 9 on top. Peek returns 9.

Practice

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

easy

A. Shows the bottom item without removing it

B. Adds a new item to the bottom of the stack

C. Removes and returns the top item from the stack

D. Clears all items from the stack

Solution

1. Step 1: Understand the pop operation

   The pop operation removes the last item added to the stack, which is the top item.

2. Step 2: Confirm the behavior of pop

   It returns this top item and removes it from the stack, following last-in, first-out order.

3. Final Answer:

   Removes and returns the top item from the stack -> Option C

4. Quick Check:

   pop = remove top item [OK]

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

Common Mistakes:

• Thinking pop adds an item
• Confusing pop with peek
• Assuming pop removes bottom item

2. Which of the following is the correct way to add an item 5 to a stack named s using the push operation?

easy

A. s.pop(5)

B. s.push(5)

C. s.peek(5)

D. s.add(5)

Solution

1. Step 1: Identify the push syntax

   Push adds an item to the top of the stack, usually called with push(item).

2. Step 2: Match the correct method call

   Only s.push(5) correctly adds 5 to the stack.

3. Final Answer:

   s.push(5) -> Option B

4. Quick Check:

   push = add item [OK]

Hint: Push means add, so look for push() method [OK]

Common Mistakes:

• Using pop instead of push
• Using peek to add items
• Using add which is not standard for stacks

3. Given the following stack operations:

s.push(10)
s.push(20)
s.pop()
s.push(30)
top = s.peek()

What is the value of top after these operations?

medium

A. 30

B. 20

C. None

D. 10

Solution

1. Step 1: Trace the stack changes step-by-step

   After push(10), stack = [10]. After push(20), stack = [10, 20]. After pop(), top 20 is removed, stack = [10]. After push(30), stack = [10, 30].

2. Step 2: Determine the top item with peek

   Peek returns the top item without removing it, so top = 30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Last pushed after pop is 30 [OK]

Hint: Track stack after each operation carefully [OK]

Common Mistakes:

• Forgetting pop removes top
• Assuming peek removes item
• Mixing order of operations

4. Consider this code snippet for a stack:

stack = []
stack.push(1)
stack.push(2)
item = stack.pop()

What is the error in this code?

medium

A. stack is not initialized correctly

B. pop() does not return the top item

C. stack.pop() removes the bottom item, not top

D. push() is not a method of list in Python

Solution

1. Step 1: Check stack initialization

   The stack is initialized as an empty list, which is correct in Python.

2. Step 2: Verify method usage on list

   Python lists use append() to add items, not push(). Using push() causes an error.

3. Final Answer:

   push() is not a method of list in Python -> Option D

4. Quick Check:

   Python list uses append(), not push() [OK]

Hint: Remember Python list uses append(), not push() [OK]

Common Mistakes:

• Assuming push() works on Python lists
• Confusing pop() behavior
• Thinking stack initialization is wrong

5. You want to reverse a list [1, 2, 3] using a stack. Which sequence of stack operations correctly reverses the list?

hard

A. Push all items in order, then pop all items to get reversed list

B. Pop all items first, then push them back in order

C. Peek all items, then push them again

D. Push items in reverse order, then pop all items

Solution

1. Step 1: Understand stack reversal logic

   Pushing items in original order stores them with last item on top. Popping all items removes them in reverse order.

2. Step 2: Apply push then pop to reverse list

   Push 1, then 2, then 3. Popping returns 3, 2, 1, which is the reversed list.

3. Final Answer:

   Push all items in order, then pop all items to get reversed list -> Option A

4. Quick Check:

   Push then pop reverses order [OK]

Hint: Push in order, pop all to reverse list [OK]

Common Mistakes:

• Pushing in reverse order first
• Using peek to reverse
• Popping before pushing