Queue vs Stack When to Use Which in DSA Java - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the top of a stack.

DSA Java

Stack<Integer> stack = new Stack<>();
stack.[1](10);

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

Apush

Badd

Coffer

Denqueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'add' which works for lists but is not the standard stack method.

Using 'offer' or 'enqueue' which are queue methods.

The push method adds an element to the top of the stack.

2fill in blank

medium

Complete the code to remove an element from the front of a queue.

DSA Java

Queue<String> queue = new LinkedList<>();
queue.add("apple");
String item = queue.[1]();

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

Apop

Bpeek

Cpush

Dpoll

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pop' which is for stacks, not queues.

Using 'peek' which only views the element without removing.

The poll method removes and returns the front element of the queue.

3fill in blank

hard

Fix the error in the code to check if a stack is empty before popping.

DSA Java

Stack<Double> stack = new Stack<>();
if (!stack.[1]()) {
    stack.pop();
}

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

Asize

Bempty

CisEmpty

Dcontains

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'empty()' which is a legacy method and less common.

Using 'size()' which returns a number, not a boolean.

The isEmpty() method checks if the stack has no elements before popping.

4fill in blank

hard

Fill both blanks to create a queue and add an element to it.

DSA Java

Queue<String> queue = new [1]<>();
queue.[2]("banana");

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

ALinkedList

Bpush

Cadd

DStack

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Stack' instead of 'LinkedList' for a queue.

Using 'push' which is for stacks, not queues.

Use LinkedList to create a queue and add to insert an element.

5fill in blank

hard

Fill all three blanks to create a stack, push an element, and then pop it.

DSA Java

Stack<Integer> stack = new [1]<>();
stack.[2](5);
int value = stack.[3]();

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

AStack

Bpush

Cpop

DLinkedList

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'LinkedList' to create a stack instead of 'Stack'.

Using 'pop' before 'push' causing empty stack error.

Create a stack with Stack, add with push, and remove with pop.

Practice

1. Which data structure should you use when you need to process elements in the exact order they arrive, like customers waiting in line?

easy

A. Stack

B. Queue

C. Array

D. HashMap

Solution

1. Step 1: Understand the order requirement

   Processing elements in the exact order they arrive means first-in, first-out (FIFO).

2. Step 2: Match data structure behavior

   Queue follows FIFO, so it processes elements in the order they come.

3. Final Answer:

   Queue -> Option B

4. Quick Check:

   FIFO = Queue [OK]

Hint: Use Queue for first-in, first-out order [OK]

Common Mistakes:

• Confusing Stack (LIFO) with Queue (FIFO)
• Choosing Array without order guarantees
• Using HashMap which has no order

2. Which of the following is the correct way to add an element to a Stack in Java?

easy

A. stack.push(element);

B. stack.enqueue(element);

C. stack.addLast(element);

D. stack.insert(element);

Solution

1. Step 1: Recall Stack method to add elements

   In Java, Stack uses push() to add elements on top.

2. Step 2: Check other options

   enqueue() is for Queue, addLast() is for Deque, insert() is not a Stack method.

3. Final Answer:

   stack.push(element); -> Option A

4. Quick Check:

   Stack add = push() [OK]

Hint: Use push() to add elements to Stack [OK]

Common Mistakes:

• Using enqueue() which is for Queue
• Using addLast() which is for Deque
• Using insert() which does not exist in Stack

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

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

medium

A. 30
10

B. 10
20

C. 30
20

D. 20
30

Solution

1. Step 1: Trace push operations

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

2. Step 2: Trace pop and peek

   pop() removes and returns top: 30. peek() returns new top without removing: 20.

3. Final Answer:

   30\n20 -> Option C

4. Quick Check:

   pop = 30, peek = 20 [OK]

Hint: pop() removes top, peek() shows top without removing [OK]

Common Mistakes:

• Confusing pop() and peek() results
• Assuming FIFO order instead of LIFO
• Mixing push order with pop output

4. Identify the error in this Java code snippet using a Queue:

import java.util.Queue;
import java.util.LinkedList;
public class Test {
  public static void main(String[] args) {
    Queue<Integer> queue = new LinkedList<>();
    queue.push(5);
    queue.add(10);
    System.out.println(queue.poll());
  }
}

medium

A. Queue interface has no push() method

B. LinkedList cannot be used as Queue

C. poll() does not remove elements

D. add() method is invalid for Queue

Solution

1. Step 1: Check Queue methods

   Queue interface does not have push() method; push() is for Stack.

2. Step 2: Validate other methods

   LinkedList can be used as Queue, poll() removes and returns head, add() is valid for Queue.

3. Final Answer:

   Queue interface has no push() method -> Option A

4. Quick Check:

   Queue push() invalid [OK]

Hint: Queue uses add(), not push() [OK]

Common Mistakes:

• Using push() on Queue instead of add()
• Thinking poll() does not remove element
• Believing LinkedList can't implement Queue

5. You need to design a system that tracks the most recent 5 user actions to allow undo operations. Which data structure is best and why?

hard

A. Queue, because it processes actions in order

B. HashMap, because it stores actions with keys

C. ArrayList, because it allows random access

D. Stack, because it reverses the order for undo

Solution

1. Step 1: Understand undo operation behavior

   Undo requires reversing the order of actions: last action undone first.

2. Step 2: Match data structure to behavior

   Stack follows last-in, first-out (LIFO), perfect for undo functionality.

3. Final Answer:

   Stack, because it reverses the order for undo -> Option D

4. Quick Check:

   Undo = LIFO = Stack [OK]

Hint: Undo needs LIFO order, so use Stack [OK]

Common Mistakes:

• Choosing Queue which is FIFO, not suitable for undo
• Using HashMap which has no order
• Thinking ArrayList automatically reverses order