Enqueue Operation in DSA Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the enqueue operation do in a queue?

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

intermediate

In Rust, which data structure is commonly used to implement a queue for efficient enqueue and dequeue operations?

The VecDeque from the standard library is commonly used because it allows adding and removing elements efficiently from both ends.

intermediate

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

The enqueue operation will fail or block because there is no space to add the new element until some elements are dequeued.

intermediate

Explain the time complexity of the enqueue operation in a queue implemented with a linked list.

The enqueue operation takes O(1) time because it adds the new element at the tail without traversing the list.

beginner

What is the difference between enqueue and push operations?

Enqueue adds an element to the end of a queue (FIFO), while push adds an element to the top of a stack (LIFO).

What is the correct position to add an element during enqueue in a queue?

A At the middle of the queue

B At the front of the queue

C At the end (rear) of the queue

D At any random position

Enqueue always adds the element at the end (rear) of the queue to maintain FIFO order.

Which Rust standard library collection is best suited for implementing a queue?

A VecDeque

B HashMap

C LinkedList

D BinaryHeap

VecDeque allows efficient insertion and removal from both ends, ideal for queue operations.

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

A O(n)

B O(1)

C O(log n)

D O(n^2)

Adding at the tail of a linked list is done in constant time, O(1).

If a queue is empty, what happens when you enqueue an element?

A The element is added at the front and rear simultaneously

B The operation fails

C The queue deletes itself

D The element becomes the first and only element

When the queue is empty, the new element becomes both the front and rear element.

Which of these is NOT true about enqueue operation?

A It removes an element from the queue

B It follows FIFO order

C It adds an element at the rear

D It can fail if the queue is full

Enqueue adds elements; removing elements is done by dequeue.

Describe the enqueue operation in a queue and how it maintains the FIFO principle.

Think about where new elements go and the order they come out.

Explain how you would implement enqueue in Rust using VecDeque and why it is efficient.

Consider which method adds to the end and why VecDeque is preferred.

Practice

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

easy

A. Returns the number of elements in the queue

B. Removes an element from the front of the queue

C. Checks if the queue is empty

D. Adds an element to the end of the queue

Solution

1. Step 1: Understand 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 back of the queue.

3. Final Answer:

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

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 queue size

2. Which of the following is the correct Rust syntax to enqueue an element 5 into a queue implemented as a VecDeque named queue?

easy

A. queue.push_front(5);

B. queue.enqueue(5);

C. queue.push_back(5);

D. queue.insert(5);

Solution

1. Step 1: Identify VecDeque enqueue method

   In Rust, VecDeque uses push_back to add elements at the end.

2. Step 2: Check options for correct method

   Only queue.push_back(5); correctly adds 5 at the back.

3. Final Answer:

   queue.push_back(5); -> Option C

4. Quick Check:

   VecDeque enqueue = push_back [OK]

Hint: Use push_back to enqueue in VecDeque [OK]

Common Mistakes:

• Using push_front instead of push_back
• Using non-existent enqueue method
• Using insert without index

3. What will be the output of the following Rust code snippet?

use std::collections::VecDeque;

fn main() {
    let mut queue = VecDeque::new();
    queue.push_back(10);
    queue.push_back(20);
    queue.push_back(30);
    queue.push_back(40);
    println!("{:?}", queue);
}

medium

A. [10, 20, 30, 40]

B. [40, 30, 20, 10]

C. [10, 20, 30]

D. Compilation error

Solution

1. Step 1: Trace enqueue operations

   Elements 10, 20, 30, 40 are added in order at the back of the queue.

2. Step 2: Understand VecDeque print format

   Printing the queue shows elements in insertion order inside square brackets.

3. Final Answer:

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

4. Quick Check:

   Enqueue order preserved in VecDeque [OK]

Hint: VecDeque prints elements in insertion order [OK]

Common Mistakes:

• Assuming push_back adds at front
• Confusing output order
• Expecting compilation error

4. Identify the error in the following Rust code that tries to enqueue an element into a queue:

use std::collections::VecDeque;

fn main() {
    let mut queue = VecDeque::new();
    queue.enqueue(15);
    println!("{:?}", queue);
}

medium

A. VecDeque cannot store integers directly

B. VecDeque has no method named 'enqueue'

C. println! macro syntax is incorrect

D. Missing 'mut' keyword for queue variable

Solution

1. Step 1: Check VecDeque methods

   VecDeque does not have an 'enqueue' method; it uses 'push_back' to add elements.

2. Step 2: Verify other code parts

   The queue is mutable and println! syntax is correct; VecDeque supports integers.

3. Final Answer:

   VecDeque has no method named 'enqueue' -> Option B

4. Quick Check:

   Use push_back, not enqueue [OK]

Hint: VecDeque uses push_back, not enqueue [OK]

Common Mistakes:

• Using enqueue method on VecDeque
• Forgetting mut keyword (not error here)
• Misreading println! syntax

5. You want to implement a queue using a Rust Vec where enqueue adds elements at the end and dequeue removes from the front. Which of the following code snippets correctly enqueues the element 7?

hard

A. queue.push(7);

B. queue.insert(0, 7);

C. queue.remove(0); queue.push(7);

D. queue.pop(); queue.push(7);

Solution

1. Step 1: Understand enqueue in Vec-based queue

   Enqueue means adding at the end, so push adds element at the back.

2. Step 2: Analyze options

   queue.push(7); uses push(7) which correctly adds 7 at the end. Others either insert at front or remove elements.

3. Final Answer:

   queue.push(7); -> Option A

4. Quick Check:

   Vec enqueue = push at end [OK]

Hint: Use push to enqueue in Vec-based queue [OK]

Common Mistakes:

• Using insert(0, val) which adds at front
• Removing elements before enqueue
• Using pop which removes last element