Enqueue Operation in DSA Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the enqueue operation do in a queue?

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

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 when the queue is full (in a fixed-size queue)?

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

beginner

Show the state of the queue after enqueueing 5 into an empty queue.

Queue state: 5 -> null (5 is the only element at the front and rear).

beginner

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

It models adding people or items to the end of a line, ensuring fair order of service.

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

A At the rear (end) of the queue

B At the front (start) of the queue

C In the middle of the queue

D It removes an element

Enqueue always adds the new element at the rear of the queue.

What is the order principle that a queue follows?

A Last-In-First-Out (LIFO)

B Sorted order

C Random order

D First-In-First-Out (FIFO)

Queues follow FIFO, meaning the first element added is the first to be removed.

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

A The enqueue operation fails or waits

B The queue expands automatically

C The element is added anyway

D The front element is removed automatically

In a fixed-size queue, enqueue fails or waits if there is no space.

If a queue is empty, what will be the state after enqueueing element 10?

A null -> 10

B 10 -> null

C Empty queue

D 10 -> 10

After enqueue, the queue has one element 10 pointing to null.

Which real-life example best represents the enqueue operation?

A Jumping the queue

B Taking a book off a shelf

C Adding a person to the end of a line

D Randomly picking a card

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

Explain the enqueue operation in a queue and describe what happens to the queue's state after enqueueing an element.

Think about adding a new person to the end of a line.

Describe what happens when you try to enqueue an element into a full fixed-size queue and why this matters.

Imagine a line that cannot grow any longer.

Practice

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

easy

A. Removes an element from the front of the queue

B. Checks if the queue is empty

C. Adds an element to the end of the queue

D. Returns the size of the queue

Solution

1. Step 1: Understand the queue structure

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

2. Step 2: Define enqueue operation

   Enqueue means adding a new element to the back of the queue.

3. Final Answer:

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

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
• Thinking enqueue removes elements
• Mixing enqueue with checking size

2. Which of the following is the correct syntax to enqueue an element 5 into a queue named q in Go?

easy

A. q.Enqueue(5)

B. q.enqueue(5)

C. enqueue(q, 5)

D. q.add(5)

Solution

1. Step 1: Identify method naming conventions in Go

   Methods in Go are usually capitalized if exported, so Enqueue is correct.

2. Step 2: Check method call on queue instance

   Calling q.Enqueue(5) means calling method Enqueue on queue q with argument 5.

3. Final Answer:

   q.Enqueue(5) -> Option A

4. Quick Check:

   Method call with capital Enqueue = q.Enqueue(5) [OK]

Hint: Use capitalized method names for exported functions in Go [OK]

Common Mistakes:

• Using lowercase method names incorrectly
• Calling enqueue as a standalone function
• Using wrong method name like add

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

package main

import "fmt"

type Queue struct {
    items []int
}

func (q *Queue) Enqueue(val int) {
    q.items = append(q.items, val)
}

func (q *Queue) Print() {
    for _, v := range q.items {
        fmt.Print(v, " -> ")
    }
    fmt.Println("null")
}

func main() {
    q := Queue{}
    q.Enqueue(10)
    q.Enqueue(20)
    q.Enqueue(30)
    q.Print()
}

medium

A. Compilation error

B. 10 -> 20 -> 30 -> null

C. null

D. 30 -> 20 -> 10 -> null

Solution

1. Step 1: Understand enqueue implementation

   Enqueue appends new values at the end of the slice, so order is preserved as inserted.

2. Step 2: Trace the Print method output

   Print prints all items in order separated by ' -> ' and ends with 'null'.

3. Final Answer:

   10 -> 20 -> 30 -> null -> Option B

4. Quick Check:

   Enqueue appends, so output is 10 -> 20 -> 30 -> null [OK]

Hint: Enqueue appends at end, print shows order inserted [OK]

Common Mistakes:

• Thinking enqueue adds at front
• Confusing print order
• Assuming code has errors

4. Identify the error in the following Go code for enqueue operation:

package main

import "fmt"

type Queue struct {
    items []int
}

func (q Queue) Enqueue(val int) {
    q.items = append(q.items, val)
}

func main() {
    q := Queue{}
    q.Enqueue(5)
    fmt.Println(q.items)
}

medium

A. Enqueue method should have pointer receiver to modify queue

B. items slice should be initialized with make

C. fmt.Println is not imported

D. Append cannot be used on slices

Solution

1. Step 1: Check method receiver type

   The Enqueue method uses a value receiver, so changes to q.items do not affect original queue.

2. Step 2: Understand pointer receiver necessity

   To modify the original queue, Enqueue must have pointer receiver (q *Queue).

3. Final Answer:

   Enqueue method should have pointer receiver to modify queue -> Option A

4. Quick Check:

   Pointer receiver needed to modify struct [OK]

Hint: Use pointer receiver to modify struct fields [OK]

Common Mistakes:

• Using value receiver and expecting changes
• Forgetting to import fmt (not main error here)
• Thinking append can't be used on slices

5. You want to implement a queue using a linked list in Go. Which step is essential in the Enqueue operation to maintain the queue correctly?

hard

A. Add new node at the head and do not update head pointer

B. Add new node at the head and update tail pointer

C. Add new node at the tail but do not update tail pointer

D. Add new node at the tail and update tail pointer

Solution

1. Step 1: Understand linked list queue structure

   Queue adds new elements at the tail (end) and removes from head (front).

2. Step 2: Enqueue operation details

   To enqueue, add new node at tail and update tail pointer to new node to keep track.

3. Final Answer:

   Add new node at the tail and update tail pointer -> Option D

4. Quick Check:

   Enqueue adds at tail and updates tail pointer [OK]

Hint: Enqueue adds at tail and updates tail pointer [OK]

Common Mistakes:

• Adding at head instead of tail
• Not updating tail pointer after enqueue
• Confusing head and tail roles