Peek Top Element of Stack in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'peek' operation do in a stack?

The 'peek' operation shows the top element of the stack without removing it. It lets you see what's on top without changing the stack.

beginner

Why is 'peek' useful in stack operations?

Peek helps check the top item before deciding to pop or push. It is useful for decision-making without changing the stack.

beginner

What happens if you peek an empty stack?

Peeking an empty stack usually returns None or raises an error because there is no top element to show.

beginner

Code snippet: How to peek the top element in a Python list used as a stack?

stack = [1, 2, 3]
top = stack[-1]

Using stack[-1] gets the last item in the list, which is the top of the stack. Here, top will be 3.

beginner

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

'Pop' removes and returns the top element, changing the stack. 'Peek' only returns the top element without removing it.

What does the peek operation return in a stack?

A The top element without removing it

B The bottom element

C Removes the top element

D Removes the bottom element

Peek returns the top element without removing it from the stack.

What happens if you peek an empty stack?

A Removes the top element

B Returns the bottom element

C Returns None or error

D Adds a new element

Peeking an empty stack returns None or causes an error because there is no top element.

Which Python list index gives the top element of a stack implemented as a list?

A stack[-1]

B stack[0]

C stack[1]

D stack[len(stack)]

stack[-1] accesses the last element, which is the top of the stack.

What is the difference between peek and pop?

A Peek removes top, pop does not

B Pop removes top, peek does not

C Both remove top element

D Both do not remove top element

Pop removes and returns the top element; peek only returns it without removing.

Why might you use peek before pop?

A To clear the stack

B To remove the bottom element

C To add a new element

D To check the top element before removing

Peek lets you see the top element before deciding to remove it with pop.

Describe how the peek operation works on a stack and why it is useful.

Think about checking the top without changing the stack.

Explain what happens when you peek an empty stack and how to handle it safely.

Consider what the stack looks like when empty.

Practice

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

easy

A. Returns the bottom element of the stack

B. Removes the top element from the stack

C. Adds a new element to the top of the stack

D. Returns the top element without removing it

Solution

1. Step 1: Understand stack operations

   A stack follows Last-In-First-Out (LIFO). The top element is the last added item.

2. Step 2: Define peek operation

   Peek returns the top element without removing it, unlike pop which removes it.

3. Final Answer:

   Returns the top element without removing it -> Option D

4. Quick Check:

   Peek = top element without removal [OK]

Hint: Peek shows top without removing it [OK]

Common Mistakes:

• Confusing peek with pop
• Thinking peek adds elements
• Assuming peek returns bottom element

2. Which of the following is the correct syntax to peek the top element from a stack implemented as a Python list named stack?

easy

A. stack[-1]

B. stack.peek()

C. stack.top()

D. stack.pop()

Solution

1. Step 1: Recall Python list indexing

   Python lists use negative indexing; stack[-1] accesses the last element.

2. Step 2: Differentiate peek from pop

   stack.pop() removes the element, so it is not peek. stack.peek() and stack.top() are not valid list methods.

3. Final Answer:

   stack[-1] -> Option A

4. Quick Check:

   Peek = stack[-1] [OK]

Hint: Use stack[-1] to peek top element in Python list [OK]

Common Mistakes:

• Using pop() which removes element
• Using non-existent peek() or top() methods
• Using positive index 0 instead of -1

3. What will be the output of this code?

stack = [10, 20, 30]
print(stack[-1])
stack.append(40)
print(stack[-1])

medium

A. 30\n40

B. 10\n40

C. 30\n30

D. 40\n40

Solution

1. Step 1: Check initial peek

   Initial stack is [10, 20, 30]. stack[-1] is 30, so first print outputs 30.

2. Step 2: Append and peek again

   After appending 40, stack becomes [10, 20, 30, 40]. stack[-1] is now 40, so second print outputs 40.

3. Final Answer:

   30 40 -> Option A

4. Quick Check:

   Peek before append = 30, after append = 40 [OK]

Hint: Peek shows last element; append adds new last element [OK]

Common Mistakes:

• Confusing peek with pop output
• Not updating peek after append
• Printing first element instead of last

4. Find the error in this code that tries to peek the top element of a stack implemented as a list:

stack = [5, 15, 25]
top = stack.pop[-1]
print(top)

medium

A. pop removes element, so peek is wrong

B. pop is used incorrectly with square brackets

C. stack is empty, so error occurs

D. print statement syntax is wrong

Solution

1. Step 1: Analyze pop usage

   pop is a method and should be called with parentheses, not square brackets. stack.pop[-1] is invalid syntax.

2. Step 2: Understand correct peek syntax

   To peek, use stack[-1]. To pop, use stack.pop(). Mixing them causes error.

3. Final Answer:

   pop is used incorrectly with square brackets -> Option B

4. Quick Check:

   pop() needs parentheses, not brackets [OK]

Hint: Use parentheses for pop(), brackets for indexing [OK]

Common Mistakes:

• Using pop with brackets instead of parentheses
• Confusing peek with pop
• Assuming pop[-1] is valid syntax

5. Given a stack implemented as a list, how can you safely peek the top element without causing an error if the stack is empty?

hard

A. Use stack.pop() and catch exceptions

B. Use stack[-1] directly without checks

C. Check if stack is not empty before peeking

D. Assign None to top element always

Solution

1. Step 1: Understand empty stack risk

   Accessing stack[-1] when stack is empty causes IndexError.

2. Step 2: Implement safe peek

   Check if stack length > 0 before peeking to avoid error.

3. Final Answer:

   Check if stack is not empty before peeking -> Option C

4. Quick Check:

   Peek safely by checking stack length first [OK]

Hint: Always check stack not empty before peek [OK]

Common Mistakes:

• Peeking without checking empty stack
• Using pop() which removes element
• Ignoring exceptions from empty stack access