Queue Concept and FIFO Principle in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this queue operation sequence?

Consider a queue where we enqueue numbers 1, 2, 3, then dequeue once, then enqueue 4. What is the state of the queue?

DSA Typescript

class Queue {
  private items: number[] = [];
  enqueue(item: number) { this.items.push(item); }
  dequeue() { return this.items.shift(); }
  print() { return this.items.join(' -> ') + ' -> null'; }
}

const q = new Queue();
q.enqueue(1);
q.enqueue(2);
q.enqueue(3);
q.dequeue();
q.enqueue(4);
console.log(q.print());

A 3 -> 4 -> null

B 2 -> 3 -> 4 -> null

C 1 -> 2 -> 3 -> 4 -> null

D 1 -> 3 -> 4 -> null

💡 Hint

Remember FIFO means first in, first out. The first number enqueued is removed first.

We add 1, 2, 3 in order. Dequeue removes 1 (the first). Then enqueue 4. So queue has 2, 3, 4 in order.

🧠 Conceptual

intermediate

Which property best describes a queue?

Choose the property that correctly describes how a queue works.

A First In, First Out (FIFO)

B Random Access

C Last In, First Out (LIFO)

D Sorted Order

💡 Hint

Think about who leaves first in a line.

A queue follows FIFO: the first element added is the first to be removed.

🔧 Debug

advanced

What error does this queue code produce?

This TypeScript queue code tries to dequeue from an empty queue. What error occurs?

DSA Typescript

class Queue {
  private items: number[] = [];
  dequeue() { return this.items.shift(); }
}

const q = new Queue();
console.log(q.dequeue().toString());

A TypeError: Cannot read property 'toString' of undefined

B SyntaxError: Unexpected token

C ReferenceError: q is not defined

D No error, prints 'undefined'

💡 Hint

What does shift() return if the array is empty?

shift() returns undefined if empty. Calling toString() on undefined causes TypeError.

🚀 Application

advanced

How many items remain after these queue operations?

Start with an empty queue. Enqueue 5 items, dequeue 3 items, enqueue 2 more items. How many items are in the queue now?

A 2

B 5

C 3

D 4

💡 Hint

Count carefully: add 5, remove 3, add 2.

5 enqueued - 3 dequeued = 2 left, then 2 more enqueued = 4 total.

❓ Predict Output

expert

What is the output after this complex queue sequence?

Given the queue operations below, what is printed?

DSA Typescript

class Queue {
  private items: number[] = [];
  enqueue(item: number) { this.items.push(item); }
  dequeue() { return this.items.shift(); }
  print() { return this.items.join(' -> ') + ' -> null'; }
}

const q = new Queue();
q.enqueue(10);
q.enqueue(20);
q.enqueue(30);
q.dequeue();
q.enqueue(40);
q.dequeue();
q.enqueue(50);
console.log(q.print());

A 40 -> 50 -> null

B 20 -> 30 -> 40 -> 50 -> null

C 30 -> 40 -> 50 -> null

D 10 -> 20 -> 30 -> 40 -> 50 -> null

💡 Hint

Track each enqueue and dequeue step carefully in order.

Enqueue 10,20,30; dequeue removes 10; enqueue 40; dequeue removes 20; enqueue 50; queue is 30,40,50.

Practice

1. What does the FIFO principle mean in the context of a queue?

easy

A. First In, First Out - the first element added is the first removed

B. First In, First Out - the last element added is the first removed

C. Last In, First Out - the first element added is the last removed

D. Last In, First Out - the last element added is the last removed

Solution

1. Step 1: Understand FIFO meaning

   FIFO means the first element that enters the queue is the first to leave.

2. Step 2: Match FIFO to queue behavior

   Queues follow FIFO, so the first added element is removed first.

3. Final Answer:

   First In, First Out - the first element added is the first removed -> Option A

4. Quick Check:

   FIFO = First In, First Out [OK]

Hint: Remember queue = line; first come, first served [OK]

Common Mistakes:

• Confusing FIFO with LIFO
• Thinking last added is removed first
• Mixing queue with stack behavior

2. Which of the following is the correct TypeScript syntax to create an empty queue using an array?

easy

A. let queue = {};

B. const queue: number[] = [];

C. const queue = new Queue();

D. queue = [];

Solution

1. Step 1: Identify TypeScript array syntax

   To create an empty array of numbers, use const queue: number[] = [];

2. Step 2: Check other options

   const queue = new Queue(); uses a class not defined here, A creates an object, D is missing declaration keyword.

3. Final Answer:

   const queue: number[] = []; -> Option B

4. Quick Check:

   Array declaration with type and empty brackets [OK]

Hint: Use const with type and empty brackets for empty array [OK]

Common Mistakes:

• Using object {} instead of array []
• Missing type annotation in TypeScript
• Omitting declaration keyword (const/let)

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

const queue: number[] = [];
queue.push(10);
queue.push(20);
queue.push(30);
const first = queue.shift();
console.log(queue);

medium

A. []

B. [10, 20, 30]

C. [30]

D. [20, 30]

Solution

1. Step 1: Add elements to queue

   Elements 10, 20, 30 are added to the queue in order.

2. Step 2: Remove first element using shift()

   shift() removes 10, so queue now contains [20, 30].

3. Final Answer:

   [20, 30] -> Option D

4. Quick Check:

   After shift(), queue = [20, 30] [OK]

Hint: shift() removes first element, push() adds last [OK]

Common Mistakes:

• Confusing shift() with pop()
• Expecting original array unchanged
• Thinking push() removes elements

4. Identify the error in this TypeScript queue code:

const queue: number[] = [];
queue.push(5);
queue.push(10);
const removed = queue.pop();
console.log(queue);

medium

A. Using pop() removes last, not first element in queue

B. push() is not valid for arrays

C. queue should be declared as object, not array

D. Missing type annotation for queue

Solution

1. Step 1: Understand queue removal method

   Queue removes elements in FIFO order, so first element should be removed.

2. Step 2: Check method used

   pop() removes last element, which breaks FIFO principle.

3. Final Answer:

   Using pop() removes last, not first element in queue -> Option A

4. Quick Check:

   Queue removal must use shift(), not pop() [OK]

Hint: Use shift() to remove first, pop() removes last [OK]

Common Mistakes:

• Using pop() instead of shift() for queue
• Thinking push() removes elements
• Confusing array with object declaration

5. You have a queue implemented as an array in TypeScript. You add elements 1, 2, 3, then remove two elements. What will be the state of the queue after these operations?

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

hard

A. [2, 3]

B. [1, 2, 3]

C. [3]

D. []

Solution

1. Step 1: Add elements to queue

   Elements 1, 2, 3 are added in order: queue = [1, 2, 3]

2. Step 2: Remove two elements using shift()

   First shift() removes 1, second shift() removes 2, leaving [3]

3. Final Answer:

   [3] -> Option C

4. Quick Check:

   After two shifts, queue = [3] [OK]

Hint: Each shift() removes one from front [OK]

Common Mistakes:

• Removing elements with pop() instead of shift()
• Expecting queue to be empty after two removals
• Confusing order of elements after removals