Enqueue Operation in DSA Javascript - Time & Space Complexity

Time Complexity: Enqueue Operation

O(1)

Understanding Time Complexity

We want to understand how the time to add an item to a queue changes as the queue grows.

How does the enqueue operation behave when the queue gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following enqueue code for a queue implemented with an array.

function enqueue(queue, item) {
  queue.push(item);
}
    

This code adds an item to the end of the queue using the array's push method.

Identify Repeating Operations

Look for loops or repeated steps inside the enqueue operation.

• Primary operation: Adding one item to the end of the array.
• How many times: Exactly once per enqueue call.

How Execution Grows With Input

Adding one item takes about the same time no matter how many items are already in the queue.

Input Size (n)	Approx. Operations
10	1 push operation
100	1 push operation
1000	1 push operation

Pattern observation: The time stays almost the same as the queue grows.

Final Time Complexity

Time Complexity: O(1)

This means adding an item takes constant time regardless of queue size.

Common Mistake

[X] Wrong: "Enqueue takes longer as the queue gets bigger because it has to move all items."

[OK] Correct: The push method adds at the end without moving existing items, so time does not grow with queue size.

Interview Connect

Understanding enqueue time helps you explain how queues work efficiently in real programs.

Self-Check

"What if the queue was implemented with a linked list instead of an array? How would the time complexity of enqueue change?"

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 behavior

   A queue works like a line where new people join at the end.

2. Step 2: Define enqueue operation

   Enqueue means adding a new element at the end of the queue.

3. Final Answer:

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

4. Quick Check:

   Enqueue = Add at end [OK]

Hint: Enqueue always adds at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue
• Thinking enqueue removes elements
• Mixing enqueue with checking size

2. Which of the following is the correct syntax to add an element 5 to a queue implemented as an array named queue in JavaScript?

easy

A. queue.enqueue(5);

B. queue.add(5);

C. queue.push(5);

D. queue.insert(5);

Solution

1. Step 1: Identify JavaScript array method for adding at end

   JavaScript arrays use push() to add elements at the end.

2. Step 2: Match method to enqueue operation

   Enqueue adds to the end, so queue.push(5); is correct syntax.

3. Final Answer:

   queue.push(5); -> Option C

4. Quick Check:

   Array push adds at end [OK]

Hint: Use push() to enqueue in array-based queue [OK]

Common Mistakes:

• Using non-existent methods like add() or insert()
• Confusing enqueue with dequeue methods
• Trying to use enqueue() directly on array

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

const queue = [];
queue.push(10);
queue.push(20);
queue.push(30);
console.log(queue);

medium

A. [10, 20, 30]

B. [30, 20, 10]

C. [10, 30, 20]

D. Error

Solution

1. Step 1: Trace each enqueue operation

   First push adds 10: queue = [10]. Second push adds 20: queue = [10, 20]. Third push adds 30: queue = [10, 20, 30].

2. Step 2: Print the queue array

   Logging queue prints the array as it is: [10, 20, 30].

3. Final Answer:

   [10, 20, 30] -> Option A

4. Quick Check:

   push adds at end, so order is [10, 20, 30] [OK]

Hint: push() adds at end, so order stays as inserted [OK]

Common Mistakes:

• Assuming push adds at front
• Confusing output order
• Expecting error due to empty initialization

4. Identify the error in the following enqueue implementation for a queue using an array:

class Queue {
  constructor() {
    this.items = [];
  }
  enqueue(element) {
    this.items.unshift(element);
  }
}

medium

A. enqueue method should remove element, not add

B. Using unshift adds element at the front, not the end

C. Array should be initialized as null, not []

D. Missing return statement in enqueue method

Solution

1. Step 1: Understand unshift behavior

   unshift adds element at the start (front) of the array, not the end.

2. Step 2: Compare with enqueue operation

   Enqueue must add element at the end, so unshift is incorrect here.

3. Final Answer:

   Using unshift adds element at the front, not the end -> Option B

4. Quick Check:

   Enqueue adds at end, unshift adds at front [OK]

Hint: Use push() for enqueue, not unshift() [OK]

Common Mistakes:

• Confusing unshift with push
• Thinking unshift adds at end
• Ignoring method behavior differences

5. You have a queue implemented with an array. You want to enqueue multiple elements [7, 8, 9] at once while preserving their order. Which code snippet correctly enqueues all elements in order?

hard

A. queue.unshift(7, 8, 9);

B. queue.concat([7, 8, 9]);

C. queue.push([7, 8, 9]);

D. [7, 8, 9].forEach(e => queue.push(e));

Solution

1. Step 1: Analyze each option for enqueue behavior

   queue.push([7, 8, 9]); adds the entire array as one element, not individual elements. queue.concat([7, 8, 9]); creates a new array but does not modify queue. [7, 8, 9].forEach(e => queue.push(e)); pushes each element individually, preserving order. queue.unshift(7, 8, 9); adds elements at front, reversing order.

2. Step 2: Choose the method that enqueues elements individually at the end

   Using forEach with push adds each element in order at the end of queue.

3. Final Answer:

   [7, 8, 9].forEach(e => queue.push(e)); -> Option D

4. Quick Check:

   forEach + push enqueues elements in order [OK]

Hint: Use forEach with push to enqueue multiple elements in order [OK]

Common Mistakes:

• Using push with array adds nested array
• Using unshift reverses order
• Forgetting to assign concat result back to queue