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

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of peeking the top element after push operations?

Consider the following Java code using a stack. What will be printed after the peek operation?

DSA Java

import java.util.Stack;
public class Main {
    public static void main(String[] args) {
        Stack<Integer> stack = new Stack<>();
        stack.push(10);
        stack.push(20);
        stack.push(30);
        System.out.println(stack.peek());
    }
}

A 30

B 20

C 10

D EmptyStackException

💡 Hint

Peek returns the last pushed element without removing it.

The stack pushes 10, then 20, then 30. Peek returns the top element, which is 30.

❓ Predict Output

intermediate

What happens when peek is called on an empty stack?

What error or output will this Java code produce?

DSA Java

import java.util.Stack;
public class Main {
    public static void main(String[] args) {
        Stack<Integer> stack = new Stack<>();
        System.out.println(stack.peek());
    }
}

A null

B No output

C 0

D EmptyStackException

💡 Hint

Peek on empty stack throws an exception.

Calling peek on an empty Java Stack throws EmptyStackException.

🔧 Debug

advanced

Why does this peek operation print the wrong value?

Find the bug in this code that causes peek to print 20 instead of 30.

DSA Java

import java.util.Stack;
public class Main {
    public static void main(String[] args) {
        Stack<Integer> stack = new Stack<>();
        stack.push(10);
        stack.push(20);
        stack.push(30);
        stack.pop();
        System.out.println(stack.peek());
    }
}

A The pop() removed 10, so peek returns 30

B The pop() removed 30, so peek returns 20

C The pop() removed 30, so peek returns 10

D The pop() removed 20, so peek returns 30

💡 Hint

Pop removes the top element before peek.

Pop removes the last pushed element 30, so peek returns the next top element 20.

🧠 Conceptual

advanced

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

Choose the correct time complexity for the peek operation on a stack implemented with an array or linked list.

A O(n^2) - quadratic time

B O(n) - linear time

C O(1) - constant time

D O(log n) - logarithmic time

💡 Hint

Peek accesses the top element directly without iteration.

Peek returns the top element directly, so it runs in constant time O(1).

🚀 Application

expert

After a sequence of operations, what is the top element of the stack?

Given the following operations on an empty stack, what will peek return?

DSA Java

Stack<String> stack = new Stack<>();
stack.push("apple");
stack.push("banana");
stack.pop();
stack.push("cherry");
stack.push("date");
stack.pop();
System.out.println(stack.peek());

A "cherry"

B "banana"

C "date"

D "apple"

💡 Hint

Track each push and pop carefully to find the top element.

Operations leave "cherry" at the top after the last pop removes "date".

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