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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the end of a queue.

DSA Javascript

queue.push([1]);

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

Ashift()

Bpop()

Celement

Dunshift()

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() which removes from the end.

Using shift() which removes from the front.

To add an element to the end of a queue, use push(element).

2fill in blank

medium

Complete the code to remove the first element from a queue.

DSA Javascript

const first = queue.[1]();

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

Ashift

Bpop

Cpush

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() which removes from the end.

Using push() which adds to the end.

To remove the first element from a queue, use shift().

3fill in blank

hard

Fix the error in this stack pop operation code.

DSA Javascript

const top = stack.[1]();

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

Ashift

Bpop

Cpush

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using shift() which removes from the front.

Using push() which adds elements.

Stacks remove the top element using pop() in JavaScript.

4fill in blank

hard

Fill both blanks to implement a function that uses a stack to reverse a string.

DSA Javascript

function reverseString(str) {
  const stack = [];
  for (let char of str) {
    stack.[1](char);
  }
  let reversed = '';
  while (stack.length > 0) {
    reversed += stack.[2]();
  }
  return reversed;
}

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

Apush

Bpop

Cshift

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using shift() or unshift() which are queue operations.

Mixing push and shift.

To reverse a string using a stack, push each character, then pop to build reversed string.

5fill in blank

hard

Fill all three blanks to implement a queue using an array with enqueue and dequeue functions.

DSA Javascript

class Queue {
  constructor() {
    this.items = [];
  }
  enqueue(element) {
    this.items.[1](element);
  }
  dequeue() {
    if (this.items.length === 0) return null;
    return this.items.[2]();
  }
  peek() {
    return this.items[[3]];
  }
}

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

Apush

Bpop

C0

Dshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() for dequeue which removes from end.

Using wrong index for peek.

Enqueue adds to the end with push, dequeue removes from front with shift, peek returns first element at index 0.

Practice

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

easy

A. Graph

B. Stack

C. Tree

D. Queue

Solution

1. Step 1: Understand the order of processing

   Tasks must be processed in the order they arrive, which means first-in, first-out (FIFO).

2. Step 2: Match the data structure to FIFO

   A queue follows FIFO, so it fits the requirement perfectly.

3. Final Answer:

   Queue -> Option D

4. Quick Check:

   FIFO = Queue [OK]

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

Common Mistakes:

• Choosing stack which is last-in, first-out
• Confusing queue with tree or graph
• Thinking stack can process in arrival order

2. Which of the following is the correct way to add an element to a stack in JavaScript using an array named stack?

easy

A. stack.unshift(element);

B. stack.shift(element);

C. stack.push(element);

D. stack.pop(element);

Solution

1. Step 1: Identify stack insertion method

   Stacks add elements on top, so we use push() to add at the end of the array.

2. Step 2: Check JavaScript array methods

   push() adds element at the end, shift() removes from front, unshift() adds at front, pop() removes from end.

3. Final Answer:

   stack.push(element); -> Option C

4. Quick Check:

   Stack add = push() [OK]

Hint: Use push() to add on top of stack [OK]

Common Mistakes:

• Using shift() which removes from front
• Using unshift() which adds at front (queue behavior)
• Using pop() which removes element

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

const queue = [];
queue.push(1);
queue.push(2);
queue.push(3);
const first = queue.shift();
console.log(queue);

medium

A. [1, 2, 3]

B. [2, 3]

C. [3, 2]

D. [1, 3]

Solution

1. Step 1: Trace queue operations

   Elements 1, 2, 3 are added to queue using push(). Then shift() removes the first element (1).

2. Step 2: Determine queue state after shift()

   After removing 1, queue contains [2, 3].

3. Final Answer:

   [2, 3] -> Option B

4. Quick Check:

   shift() removes first element [OK]

Hint: shift() removes first element from queue [OK]

Common Mistakes:

• Thinking push adds to front
• Confusing shift() with pop()
• Expecting original array unchanged

4. The following code is intended to implement a stack using an array, but it has a bug. What is the bug?

const stack = [];
stack.push(10);
stack.push(20);
const top = stack.shift();
console.log(top);

medium

A. Using shift() removes bottom element, not top

B. push() should be replaced with pop()

C. stack is not initialized correctly

D. Missing return statement

Solution

1. Step 1: Understand stack behavior

   Stack removes the last added element (top) using pop(), not shift().

2. Step 2: Identify incorrect method usage

   shift() removes first element (bottom), so using shift() breaks stack LIFO behavior.

3. Final Answer:

   Using shift() removes bottom element, not top -> Option A

4. Quick Check:

   Stack remove = pop(), not shift() [OK]

Hint: Use pop() to remove top from stack [OK]

Common Mistakes:

• Using shift() instead of pop()
• Thinking push() removes element
• Confusing stack with queue methods

5. You are designing a browser's back button feature. Which data structure is best to store the history of visited pages and why?

hard

A. Stack, because the last visited page should be returned first

B. Queue, because pages are visited in order and removed from front

C. Graph, because pages link to each other

D. Array, because it allows random access

Solution

1. Step 1: Understand back button behavior

   The back button returns to the last visited page first, so last-in, first-out (LIFO) order is needed.

2. Step 2: Match data structure to LIFO

   Stack follows LIFO, so it is the best choice to store history for back navigation.

3. Final Answer:

   Stack, because the last visited page should be returned first -> Option A

4. Quick Check:

   Back button = LIFO = Stack [OK]

Hint: Back button needs last-in, first-out order [OK]

Common Mistakes:

• Choosing queue which is FIFO
• Thinking graph or array fits better
• Ignoring order of page visits