Peek Top Element of Stack in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to peek the top element of the stack.

DSA Java

int topElement = stack.[1]();

Drag options to blanks, or click blank then click option'

Apop

Bempty

Cpush

Dpeek

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() removes the element from the stack.

Using push() adds an element instead of peeking.

Using empty() checks if the stack is empty, not the top element.

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

2fill in blank

medium

Complete the code to check if the stack is empty before peeking.

DSA Java

if (!stack.[1]()) {
    int top = stack.peek();
}

Drag options to blanks, or click blank then click option'

Apush

Bempty

Csearch

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() instead of empty() causes errors.

Using push() does not check emptiness.

Using search() is for finding elements, not checking emptiness.

The empty() method returns true if the stack is empty, so we use !empty() to check if it has elements before peeking.

3fill in blank

hard

Fix the error in the code to correctly peek the top element safely.

DSA Java

if (!stack.isEmpty()) {
    int top = stack.[1]();
}

Drag options to blanks, or click blank then click option'

Aempty

Bpop

Cpeek

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() removes the element.

Using empty() is a check, not a retrieval method.

Using push() adds elements, not retrieves.

Use peek() to get the top element without removing it. pop() removes the element, which is not desired here.

4fill in blank

hard

Fill both blanks to safely peek the top element or return -1 if empty.

DSA Java

int top = stack.[1]() ? -1 : stack.[2]();

Drag options to blanks, or click blank then click option'

Aempty

Bpeek

CisEmpty

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using empty() instead of isEmpty() causes errors.

Using pop() removes the element.

Using push() is incorrect here.

Use isEmpty() to check if the stack is empty, then peek() to get the top element safely.

5fill in blank

hard

Fill both blanks to create a method that returns the top element or -1 if stack is empty.

DSA Java

public int peekTop(Stack<Integer> stack) {
    return stack.[1]() ? -1 : stack.[2]();
}

Drag options to blanks, or click blank then click option'

Aempty

Bpeek

CisEmpty

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using empty() instead of isEmpty() causes errors.

Using pop() removes the element.

Returning peek() without checking emptiness may cause exceptions.

The method uses isEmpty() to check if the stack is empty and returns -1 if true; otherwise, it returns the top element using peek().

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