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

Recall & Review

beginner

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

The 'peek' operation returns the top element of the stack without removing it, allowing you to see the current top value.

beginner

In a stack, what happens if you try to peek when the stack is empty?

Peeking an empty stack usually results in an error or a special value indicating the stack is empty, because there is no top element to return.

beginner

Why is peek useful compared to pop in a stack?

Peek lets you see the top element without removing it, so you can check the value without changing the stack's contents, unlike pop which removes the top element.

beginner

Show the C++ code snippet to peek the top element of a stack named 's'.

if (!s.empty()) { int topElement = s.top(); // use topElement as needed } else { // handle empty stack case }

beginner

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

The peek operation runs in O(1) time because it directly accesses the top element without any iteration.

What does the peek operation on a stack return?

A The top element without removing it

B The bottom element of the stack

C Removes and returns the top element

D Removes the bottom element

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

What happens if you peek an empty stack?

A Returns the bottom element

B Returns an error or special value

C Returns zero always

D Removes the top element

Peeking an empty stack usually causes an error or returns a special value indicating no elements.

Which C++ stack method is used to peek the top element?

A pop()

B peek()

C top()

D front()

The top() method returns the top element without removing it.

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

A O(n^2)

B O(log n)

C O(n)

D O(1)

Peek runs in constant time O(1) because it accesses the top element directly.

Why might you use peek instead of pop?

A To check the top element without removing it

B To remove the top element

C To clear the stack

D To add a new element

Peek lets you see the top element without changing the stack.

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

Think about how peek differs from pop.

Describe how to safely peek the top element of a stack in C++.

Consider what happens if the stack has no elements.

Practice

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

easy

A. Returns the top element without removing it

B. Removes the top element from the stack

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

D. Checks if the stack is empty

Solution

1. Step 1: Understand stack operations

   A stack is a collection where elements are added or removed from the top only.

2. Step 2: Define peek operation

   The peek operation returns the top element without removing it, unlike pop which removes it.

3. Final Answer:

   Returns the top element without removing it -> Option A

4. Quick Check:

   peek() = top element [OK]

Hint: Peek shows top without removal [OK]

Common Mistakes:

• Confusing peek with pop
• Thinking peek adds elements
• Assuming peek checks emptiness

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

easy

A. stk.pop();

B. stk.peek();

C. stk.top();

D. stk.getTop();

Solution

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

   The standard method to access the top element is top().

2. Step 2: Identify correct syntax

   stk.top(); returns the top element without removing it.

3. Final Answer:

   stk.top(); -> Option C

4. Quick Check:

   stk.top() = peek top element [OK]

Hint: Use .top() to peek in C++ STL stack [OK]

Common Mistakes:

• Using .peek() which is not in C++ STL
• Using .pop() which removes element
• Using non-existent .getTop() method

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

std::stack<int> stk;
stk.push(10);
stk.push(20);
stk.push(30);
std::cout << stk.top() << std::endl;

medium

A. 30

B. 10

C. 20

D. Compilation error

Solution

1. Step 1: Trace push operations

   Elements pushed in order: 10, then 20, then 30. Stack top is last pushed element 30.

2. Step 2: Check top() output

   stk.top() returns 30 without removing it.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Last pushed = top = 30 [OK]

Hint: Top is last pushed element [OK]

Common Mistakes:

• Confusing top with bottom element
• Assuming top removes element
• Thinking output is 20 or 10

4. Identify the error in the following code snippet that tries to peek the top element of a stack named stk:

int topElement = stk.peek();

medium

A. stk should be declared as a queue

B. peek() is not a valid method in C++ STL stack

C. topElement should be a pointer

D. stk.pop() should be used instead

Solution

1. Step 1: Check method availability in C++ STL stack

   C++ STL stack does not have a method named peek().

2. Step 2: Correct method to peek top element

   The correct method is top(), so stk.top() should be used.

3. Final Answer:

   peek() is not a valid method in C++ STL stack -> Option B

4. Quick Check:

   Use stk.top() not stk.peek() [OK]

Hint: Use .top(), not .peek() in C++ STL [OK]

Common Mistakes:

• Using peek() method which doesn't exist
• Confusing pop() with peek()
• Thinking topElement must be pointer

5. Given a stack stk with elements [5, 10, 15] pushed in that order, which code snippet correctly prints the top element without removing it and then prints the stack size?

hard

A. std::cout << stk.pop() << std::endl; std::cout << stk.size() << std::endl;

B. std::cout << stk.top() << std::endl; stk.pop(); std::cout << stk.size() << std::endl;

C. std::cout << stk.peek() << std::endl; std::cout << stk.size() << std::endl;

D. std::cout << stk.top() << std::endl; std::cout << stk.size() << std::endl;

Solution

1. Step 1: Understand peek without removal

   To peek, use stk.top() which returns top element without removing it.

2. Step 2: Check stack size after peek

   Since peek does not remove, stk.size() remains 3.

3. Final Answer:

   std::cout << stk.top() << std::endl; std::cout << stk.size() << std::endl; -> Option D

4. Quick Check:

   top() no removal + size() correct [OK]

Hint: Use top() to peek, size() unchanged [OK]

Common Mistakes:

• Using pop() which removes element
• Using peek() which is invalid in C++ STL
• Printing size after pop() expecting no change