Enqueue Operation in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the enqueue operation do in a queue?

It adds an element to the end of the queue, following the First-In-First-Out (FIFO) principle.

beginner

In a queue, where is the new element added during an enqueue operation?

The new element is added at the rear (or tail) of the queue.

intermediate

What happens if you try to enqueue an element into a full queue (fixed size)?

The enqueue operation fails or returns an error because there is no space to add a new element.

beginner

TypeScript snippet: What does this enqueue method do?

enqueue(item: T): void {
  this.items.push(item);
}

It adds the item to the end of the internal array representing the queue, effectively enqueuing it.

beginner

Why is enqueue operation important in real-life scenarios like waiting lines?

Because it models adding people or tasks to the end of a line, ensuring fair order of service.

Where does the enqueue operation add a new element in a queue?

A At the front (start) of the queue

B At the rear (end) of the queue

C In the middle of the queue

D It removes an element instead

Enqueue always adds the new element at the rear (end) of the queue.

What is the main principle followed by the enqueue operation in a queue?

A First-In-First-Out (FIFO)

B Priority order

C Random order

D Last-In-First-Out (LIFO)

Queues follow FIFO, so enqueue adds at the end and dequeue removes from the front.

What happens if you enqueue an element into a full fixed-size queue?

A The queue doubles its size automatically

B The queue removes the first element automatically

C The element is added anyway

D The enqueue operation fails or returns an error

A fixed-size queue cannot add more elements than its capacity, so enqueue fails.

In TypeScript, which array method is commonly used to enqueue an element in a queue implemented with an array?

A pop()

B shift()

C push()

D unshift()

push() adds an element to the end of the array, matching enqueue behavior.

Which real-life example best illustrates the enqueue operation?

A Adding a new customer to the end of a checkout line

B Skipping a line

C Taking a book off a shelf

D Removing the first person in line

Enqueue is like adding a new customer to the end of a waiting line.

Explain the enqueue operation in a queue and how it maintains order.

Think about how people join a line.

Describe what happens internally when you enqueue an element in a queue implemented with an array in TypeScript.

Consider how arrays grow when you add items.

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