Enqueue Operation in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after enqueue operations?

Consider a queue implemented using an array. After performing the following enqueue operations, what is the printed state of the queue?

DSA Typescript

class Queue {
  private items: number[] = [];

  enqueue(element: number) {
    this.items.push(element);
  }

  print() {
    console.log(this.items.join(' -> ') + ' -> null');
  }
}

const queue = new Queue();
queue.enqueue(5);
queue.enqueue(10);
queue.enqueue(15);
queue.print();

A 15 -> 10 -> 5 -> null

B 5 -> 10 -> 15 -> null

C null -> 5 -> 10 -> 15

D 5 -> 10 -> null

💡 Hint

Remember that enqueue adds elements to the end of the queue.

The enqueue operation adds elements to the end of the queue. So after adding 5, 10, and 15 in order, the queue will be 5 -> 10 -> 15 -> null.

❓ Predict Output

intermediate

What is the queue state after these enqueue calls?

Given this queue implementation, what will be the output after these enqueue operations?

DSA Typescript

class Queue {
  private items: number[] = [];

  enqueue(element: number) {
    this.items.unshift(element);
  }

  print() {
    console.log(this.items.join(' -> ') + ' -> null');
  }
}

const queue = new Queue();
queue.enqueue(1);
queue.enqueue(2);
queue.enqueue(3);
queue.print();

A null -> 1 -> 2 -> 3

B 1 -> 2 -> 3 -> null

C 1 -> 3 -> 2 -> null

D 3 -> 2 -> 1 -> null

💡 Hint

unshift adds elements to the front of the array.

Using unshift adds elements to the front, so the last enqueued element appears first. The queue becomes 3 -> 2 -> 1 -> null.

🔧 Debug

advanced

Why does this enqueue method leave the queue empty?

This enqueue method is intended to add elements to the queue. Why does the queue remain empty after these operations?

DSA Typescript

class Queue {
  private items: number[] = [];

  enqueue(element: number) {
    this.items[this.items.length] = element;
    this.items.length = this.items.length - 1;
  }
}

const queue = new Queue();
queue.enqueue(7);
queue.enqueue(8);

A Because reducing length after adding removes the last element, so queue is empty

B Because assigning to items[length] is invalid and causes an error

C Because enqueue does not add elements at all due to missing push

D Because the queue array is not initialized properly

💡 Hint

Check what happens when you reduce the array length after adding an element.

After adding an element at the end, reducing the length removes the last element, effectively undoing the enqueue. This causes the queue to remain empty.

🧠 Conceptual

advanced

What is the time complexity of enqueue in a linked list queue?

If a queue is implemented using a linked list with pointers to head and tail, what is the time complexity of the enqueue operation?

A O(log n) because of pointer updates

B O(n) because we must traverse the list to find the tail

C O(1) because we add at the tail directly

D O(n^2) because of repeated traversals

💡 Hint

Think about how having a tail pointer helps.

With a tail pointer, enqueue adds a new node at the tail in constant time, so the complexity is O(1).

🚀 Application

expert

After these enqueue and dequeue operations, what is the queue state?

Given this queue implementation, what is the printed queue state after the operations?

DSA Typescript

class Queue {
  private items: number[] = [];

  enqueue(element: number) {
    this.items.push(element);
  }

  dequeue() {
    return this.items.shift();
  }

  print() {
    console.log(this.items.join(' -> ') + ' -> null');
  }
}

const queue = new Queue();
queue.enqueue(1);
queue.enqueue(2);
queue.enqueue(3);
queue.dequeue();
queue.enqueue(4);
queue.print();

A 2 -> 3 -> 4 -> null

B 1 -> 2 -> 3 -> 4 -> null

C 3 -> 4 -> null

D 4 -> null

💡 Hint

Dequeue removes from the front, enqueue adds to the end.

After enqueueing 1, 2, 3, the queue is 1 -> 2 -> 3 -> null. Dequeue removes 1, leaving 2 -> 3 -> null. Enqueue 4 adds at the end, resulting in 2 -> 3 -> 4 -> null.

Practice

1. What does the enqueue operation do in a queue data structure?

easy

A. Adds an element to the end of the queue

B. Removes an element from the front of the queue

C. Checks if the queue is empty

D. Returns the size of the queue

Solution

1. Step 1: Understand the queue structure

   A queue works like a line where elements are added at the back and removed from the front.

2. Step 2: Define the enqueue operation

   Enqueue means adding a new element to the end of the queue, so it waits its turn.

3. Final Answer:

   Adds an element to the end of the queue -> Option A

4. Quick Check:

   Enqueue = Add at end [OK]

Hint: Enqueue means add at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue (removal)
• Thinking enqueue checks queue size
• Assuming enqueue removes elements

2. Which of the following is the correct TypeScript syntax to add an element 5 to a queue represented by an array queue?

easy

A. queue.pop(5);

B. queue.push(5);

C. queue.shift(5);

D. queue.unshift(5);

Solution

1. Step 1: Identify array methods for queue operations

   In TypeScript, push adds an element at the end, which matches enqueue.

2. Step 2: Match enqueue with correct method

   pop removes from end, shift removes from front, unshift adds at front. So only push adds at the end.

3. Final Answer:

   queue.push(5); -> Option B

4. Quick Check:

   Enqueue uses push() to add at end [OK]

Hint: Use push() to add at end of array for enqueue [OK]

Common Mistakes:

• Using pop() which removes instead of adds
• Using shift() or unshift() which affect front of queue
• Confusing push() with unshift()

3. What will be the state of the queue after executing the following TypeScript code?

let queue: number[] = [10, 20, 30];
queue.push(40);
queue.push(50);

medium

A. [10, 20, 30, 40, 50]

B. [50, 40, 10, 20, 30]

C. [10, 20, 30]

D. [40, 50]

Solution

1. Step 1: Start with initial queue

   The queue starts as [10, 20, 30].

2. Step 2: Apply enqueue operations

   First, push(40) adds 40 at the end: [10, 20, 30, 40]. Then push(50) adds 50 at the end: [10, 20, 30, 40, 50].

3. Final Answer:

   [10, 20, 30, 40, 50] -> Option A

4. Quick Check:

   push adds at end, final queue = [10,20,30,40,50] [OK]

Hint: push() adds elements at the end in order [OK]

Common Mistakes:

• Thinking push adds at front
• Ignoring second push operation
• Confusing push with pop or shift

4. Identify the error in the following TypeScript code that tries to enqueue an element 25 to a queue represented by an array queue:

let queue: number[] = [5, 10, 15];
queue.pop(25);

medium

A. The element 25 should be added using shift

B. pop adds element at the front, so wrong method

C. The queue array is not initialized properly

D. pop does not take arguments and removes from the end

Solution

1. Step 1: Understand pop() method behavior

   pop() removes the last element from the array and does not accept any arguments.

2. Step 2: Identify misuse of pop()

   Calling pop(25) is incorrect because pop() cannot add elements; it only removes.

3. Final Answer:

   pop does not take arguments and removes from the end -> Option D

4. Quick Check:

   pop() removes, push() adds [OK]

Hint: pop() removes last element; push() adds at end [OK]

Common Mistakes:

• Using pop() to add elements
• Confusing shift() and pop()
• Passing arguments to pop()

5. You have a queue implemented as an array in TypeScript. You want to enqueue multiple elements [100, 200, 300] at once while preserving order. Which code snippet correctly enqueues all elements in order?

hard

A. queue.unshift(100, 200, 300);

B. queue.push([100, 200, 300]);

C. queue.push(100, 200, 300);

D. queue.pop(100); queue.pop(200); queue.pop(300);

Solution

1. Step 1: Understand push with multiple arguments

   push() can take multiple arguments and adds them in order at the end of the array.

2. Step 2: Analyze each option

   queue.push(100, 200, 300); adds 100, 200, 300 in order at the end. queue.push([100, 200, 300]); adds a single array element, nesting the array. queue.unshift(100, 200, 300); adds elements at the front, reversing order. queue.pop(100); queue.pop(200); queue.pop(300); uses pop which removes elements, not adds.

3. Final Answer:

   queue.push(100, 200, 300); -> Option C

4. Quick Check:

   push(...items) adds all at end in order [OK]

Hint: Use push with multiple args to enqueue many elements [OK]

Common Mistakes:

• Using push with array argument nests array
• Using unshift adds at front, reversing order
• Using pop to add elements