Peek Top Element of Stack in DSA Java - 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 what is on top.

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 returns a special value (like null or exception) because there is no top element to show.

beginner

Java code snippet: How to peek the top element of a stack named 'stack'?

Use stack.peek(); This returns the top element without removing it.

intermediate

Why is peek useful compared to pop in stack operations?

Peek lets you check the top element without changing the stack, while pop removes the top element. Peek is useful when you want to look but not remove.

beginner

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

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

What does the peek operation do in a stack?

A Returns the top element without removing it

B Removes the top element

C Adds a new element to the top

D Checks if the stack is empty

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

What happens if you peek an empty stack in Java's Stack class?

A Throws EmptyStackException

B Returns 0

C Returns null

D Removes an element

Java's Stack.peek() throws EmptyStackException if the stack is empty.

Which method is used to peek the top element in Java's Stack?

A top()

B peek()

C pop()

D get()

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

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

A O(n)

B O(log n)

C O(n^2)

D O(1)

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

Why would you use peek instead of pop?

A To remove the top element

B To add a new element

C To see the top element without removing it

D To clear the stack

Peek lets you see the top element without removing it, unlike pop which removes it.

Explain how the peek operation works in a stack and what happens if the stack is empty.

Think about looking at the top card in a deck without taking it out.

Write a simple Java code snippet to peek the top element of a stack and handle the case when the stack is empty.

Use try-catch to avoid errors when stack is empty.

Practice

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

easy

A. Checks if the stack is empty

B. Removes and returns the top element

C. Adds a new element to the top

D. Returns the top element without removing it

Solution

1. Step 1: Understand the peek method purpose

   The peek method is designed to look at the top element of the stack without changing the stack.

2. Step 2: Compare with other stack methods

   Unlike pop, peek does not remove the element; it only returns 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 checks emptiness

2. Which of the following is the correct syntax to peek the top element of a Java Stack named stack?

easy

A. stack.peek();

B. stack.top();

C. stack.getTop();

D. stack.pop();

Solution

1. Step 1: Recall Java Stack method names

   The Java Stack class uses peek() to view the top element without removing it.

2. Step 2: Check other options for correctness

   top() and getTop() are not valid Stack methods; pop() removes the top element.

3. Final Answer:

   stack.peek(); -> Option A

4. Quick Check:

   Correct syntax for peek is stack.peek() [OK]

Hint: Use stack.peek() to view top element [OK]

Common Mistakes:

• Using pop() instead of peek()
• Using non-existent methods like top()
• Missing parentheses in method call

3. What will be the output of the following Java code?

Stack<Integer> stack = new Stack<>();
stack.push(10);
stack.push(20);
stack.push(30);
System.out.println(stack.peek());

medium

A. 10

B. 30

C. 20

D. StackEmptyException

Solution

1. Step 1: Trace the push operations

   Elements pushed in order: 10 (bottom), 20, then 30 (top).

2. Step 2: Understand peek output

   peek() returns the top element without removing it, which is 30.

3. Final Answer:

   30 -> Option B

4. Quick Check:

   Top element after pushes = 30 [OK]

Hint: Peek shows last pushed element [OK]

Common Mistakes:

• Confusing peek with pop output
• Assuming peek returns bottom element
• Expecting exception on non-empty stack

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

Stack<String> stack = new Stack<>();
System.out.println(stack.peek());

medium

A. Throws EmptyStackException at runtime

B. Prints empty string

C. Compilation error: peek() not found

D. No error, prints null

Solution

1. Step 1: Check stack initialization and usage

   The stack is created but no elements are pushed before peek is called.

2. Step 2: Understand peek behavior on empty stack

   Calling peek() on an empty stack throws EmptyStackException at runtime.

3. Final Answer:

   Throws EmptyStackException at runtime -> Option A

4. Quick Check:

   peek() on empty stack throws exception [OK]

Hint: Peek on empty stack throws exception [OK]

Common Mistakes:

• Assuming peek returns null if empty
• Expecting compile-time error
• Thinking it prints empty string

5. Given a stack with elements [5, 10, 15] (5 at bottom, 15 at top), which sequence of operations will peek the top element without removing it and then add 20 on top?

1. stack.peek();
2. stack.push(20);
3. stack.pop();
4. stack.peek();

Select the correct order of operations to achieve this.

hard

A. 3, 1, 2

B. 2, 1

C. 1, 2

D. 1, 3, 2

Solution

1. Step 1: Peek top element without removing

   Operation 1 (stack.peek()) returns 15 without removing it.

2. Step 2: Add 20 on top

   Operation 2 (stack.push(20)) adds 20 on top, so stack becomes [5, 10, 15, 20].

3. Final Answer:

   1, 2 -> Option C

4. Quick Check:

   Peek then push = 1, 2 [OK]

Hint: Peek first, then push new element [OK]

Common Mistakes:

• Popping before peek removes top
• Pushing before peek changes top
• Incorrect operation order