Peek Top Element of Stack in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after peeking the top element?

Consider the following stack operations. What will be printed after the peek operation?

DSA Python

stack = []
stack.append(10)
stack.append(20)
stack.append(30)
print(stack[-1])

A IndexError

B 20

C 10

D 30

💡 Hint

Remember, peek means looking at the last added item without removing it.

The last element added to the stack is 30, so peek returns 30.

❓ Predict Output

intermediate

What happens when peeking an empty stack?

What error will this code produce when trying to peek the top element of an empty stack?

DSA Python

stack = []
print(stack[-1])

A IndexError

B None

C ValueError

D KeyError

💡 Hint

Think about what happens when you access an index that does not exist.

Accessing stack[-1] on an empty list raises IndexError because there is no element.

🔧 Debug

advanced

Identify the error in this peek function implementation

This function is supposed to return the top element of the stack without removing it. What error will it cause?

DSA Python

def peek(stack):
    return stack.pop()

stack = [1, 2, 3]
print(peek(stack))
print(stack)

A It returns None always

B It raises IndexError on empty stack

C It removes the top element instead of just peeking

D It causes TypeError

💡 Hint

Check what pop() does compared to peek.

pop() removes and returns the top element, so the stack changes. Peek should not remove.

❓ Predict Output

advanced

What is the output after multiple peek operations?

Given the stack operations below, what will be printed?

DSA Python

stack = []
stack.append('a')
stack.append('b')
print(stack[-1])
stack.pop()
print(stack[-1])

A b\na

B a\nb

C b\nb

D a\na

💡 Hint

Peek shows the last element. Pop removes it.

First peek shows 'b', then pop removes 'b', second peek shows 'a'.

🧠 Conceptual

expert

Why is peek operation O(1) time complexity in a stack?

Choose the best explanation for why peeking the top element of a stack is done in constant time.

A Because the stack needs to traverse all elements to find the top

B Because the top element is always at the end of the data structure and can be accessed directly

C Because peek operation removes the element which takes constant time

D Because the stack is implemented as a linked list and requires traversal

💡 Hint

Think about where the top element is stored in a stack implemented with a list.

In a stack implemented with a list, the top element is at the last index, accessible directly in O(1).