Queue vs Stack When to Use Which in DSA Java - Compare & Choose

Challenge - 5 Problems

🧠 Conceptual

intermediate

Choosing Between Queue and Stack for Task Scheduling

You have a list of tasks that must be processed in the exact order they arrive. Which data structure is best to use?

A Stack, because it processes the most recent task first.

B Queue, because it processes tasks in the order they arrive.

C Stack, because it allows random access to tasks.

D Queue, because it processes tasks in reverse order.

💡 Hint

Think about which data structure follows the 'first come, first served' rule.

A queue processes elements in the order they arrive (FIFO), which is perfect for task scheduling where order matters. A stack processes elements in reverse order (LIFO), which is not suitable here.

❓ Predict Output

intermediate

Output of Stack Operations

What is the printed output after executing the following Java code?

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.print(stack.pop() + " -> ");
        System.out.print(stack.pop() + " -> ");
        System.out.print(stack.pop() + " -> null");
    }
}

A 30 -> 20 -> 10 -> null

B 10 -> 20 -> 30 -> null

C 20 -> 30 -> 10 -> null

D 10 -> 30 -> 20 -> null

💡 Hint

Remember that stack is Last In First Out (LIFO).

The last pushed element (30) is popped first, then 20, then 10. So the output is 30 -> 20 -> 10 -> null.

❓ Predict Output

advanced

Output of Queue Operations

What is the printed output after executing the following Java code?

DSA Java

import java.util.LinkedList;
import java.util.Queue;
public class Main {
    public static void main(String[] args) {
        Queue<Integer> queue = new LinkedList<>();
        queue.add(5);
        queue.add(15);
        queue.add(25);
        System.out.print(queue.poll() + " -> ");
        System.out.print(queue.poll() + " -> ");
        System.out.print(queue.poll() + " -> null");
    }
}

A 5 -> 15 -> 25 -> null

B 25 -> 15 -> 5 -> null

C 15 -> 25 -> 5 -> null

D 5 -> 25 -> 15 -> null

💡 Hint

Remember that queue is First In First Out (FIFO).

The first added element (5) is removed first, then 15, then 25. So the output is 5 -> 15 -> 25 -> null.

🧠 Conceptual

advanced

When to Use Stack Over Queue

Which scenario is best suited for using a stack instead of a queue?

A Processing print jobs in the order they arrive.

B Serving customers in a bank line.

C Undo functionality in a text editor.

D Scheduling tasks that must run in arrival order.

💡 Hint

Think about a feature where the most recent action is reversed first.

Undo functionality requires reversing the most recent action first, which fits the Last In First Out (LIFO) behavior of a stack.

🚀 Application

expert

Choosing Data Structure for Browser History

You are designing a browser history feature that allows users to go back to the previous page. Which data structure is most appropriate and why?

A Queue, because it keeps pages in the order visited and allows going back easily.

B Stack, because it allows random access to any page in history.

C Queue, because it processes pages in reverse order.

D Stack, because it allows going back to the most recently visited page first.

💡 Hint

Think about how the 'back' button works in a browser.

The browser history 'back' button returns to the most recently visited page first, which matches the Last In First Out (LIFO) behavior of a stack.

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