Enqueue Operation in DSA C#

Mental Model

Adding an item to the end of a line so it waits its turn.

Analogy: Imagine a queue at a ticket counter where new people join at the back of the line.

front -> null
rear -> null

Dry Run Walkthrough

Input: queue initially empty, enqueue values 10, then 20, then 30

Goal: Add each new value to the end of the queue maintaining order

Step 1: enqueue 10 into empty queue

front -> [10] -> null
rear -> [10]

Why: First element becomes both front and rear

Step 2: enqueue 20 at rear

front -> 10 -> [20] -> null
rear -> [20]

Why: Add new node after rear and update rear pointer

Step 3: enqueue 30 at rear

front -> 10 -> 20 -> [30] -> null
rear -> [30]

Why: Add new node after rear and update rear pointer

Result:

front -> 10 -> 20 -> 30 -> null
rear -> [30]

Annotated Code

DSA C#

using System;

public class Node
{
    public int Data;
    public Node Next;
    public Node(int data)
    {
        Data = data;
        Next = null;
    }
}

public class Queue
{
    private Node front;
    private Node rear;

    public Queue()
    {
        front = null;
        rear = null;
    }

    public void Enqueue(int value)
    {
        Node newNode = new Node(value);
        if (rear == null) // queue empty
        {
            front = newNode; // first node is front
            rear = newNode;  // and rear
            return;
        }
        rear.Next = newNode; // link new node after rear
        rear = newNode;      // update rear to new node
    }

    public void PrintQueue()
    {
        Node current = front;
        while (current != null)
        {
            Console.Write(current.Data);
            if (current.Next != null) Console.Write(" -> ");
            current = current.Next;
        }
        Console.WriteLine(" -> null");
    }
}

public class Program
{
    public static void Main()
    {
        Queue q = new Queue();
        q.Enqueue(10);
        q.Enqueue(20);
        q.Enqueue(30);
        q.PrintQueue();
    }
}

if (rear == null) // queue empty

check if queue is empty to set front and rear

front = newNode; // first node is front rear = newNode; // and rear

initialize front and rear to new node when queue empty

rear.Next = newNode; // link new node after rear

link new node at the end of queue

rear = newNode; // update rear to new node

move rear pointer to new last node

Output

10 -> 20 -> 30 -> null

Complexity Analysis

Time: O(1) because enqueue adds node directly at rear without traversal

Space: O(1) because only one new node is created per enqueue

vs Alternative: Compared to array-based queue where shifting may be needed, linked list enqueue is faster and simpler

Edge Cases

empty queue

front and rear both point to the new node

DSA C#

if (rear == null) // queue empty

When to Use This Pattern

When you see a problem asking to add items in order at the end of a waiting line, reach for the enqueue operation because it maintains order efficiently.

Common Mistakes

Mistake: Not updating rear pointer after adding new node
Fix: Always set rear = newNode after linking it

Mistake: Not handling empty queue case separately
Fix: Check if rear is null and set front and rear to new node

Summary

Adds a new element at the end of the queue.

Use when you need to maintain order of elements arriving in a line.

Remember to update rear pointer and handle empty queue separately.

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 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 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
• Thinking enqueue removes elements
• Mixing enqueue with checking size

2. Which of the following is the correct syntax to enqueue an integer value 10 into a Queue<int> named q in C#?

easy

A. q.Enqueue(10);

B. q.Add(10);

C. q.Push(10);

D. q.Insert(10);

Solution

1. Step 1: Recall C# Queue method

   The method to add elements in Queue<T> is Enqueue.

2. Step 2: Match syntax

   Calling q.Enqueue(10); adds 10 to the queue correctly.

3. Final Answer:

   q.Enqueue(10); -> Option A

4. Quick Check:

   Use Enqueue() to add in C# Queue [OK]

Hint: Use Enqueue() method to add elements in C# Queue [OK]

Common Mistakes:

• Using Add() which is not in Queue
• Using Push() which is for stacks
• Using Insert() which is not a Queue method

3. What will be the output of the following C# code?

Queue<string> q = new Queue<string>();
q.Enqueue("apple");
q.Enqueue("banana");
q.Enqueue("cherry");
Console.WriteLine(string.Join(", ", q));

medium

A. cherry, banana, apple

B. apple, banana, cherry

C. banana, cherry, apple

D. apple, cherry, banana

Solution

1. Step 1: Trace enqueue order

   Elements are added in order: apple, then banana, then cherry.

2. Step 2: Understand queue order

   Queue keeps order of insertion, so printing shows apple, banana, cherry.

3. Final Answer:

   apple, banana, cherry -> Option B

4. Quick Check:

   Queue prints in enqueue order [OK]

Hint: Queue keeps insertion order; output matches enqueue sequence [OK]

Common Mistakes:

• Assuming reverse order output
• Mixing enqueue with stack behavior
• Confusing output order with dequeue

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

Queue<int> q = new Queue<int>();
q.Enqueue(5);
q.Enqueue(10);
q.Enqueue(15);
q.Enqueue();

medium

A. Cannot enqueue integer values

B. Queue is not initialized

C. Enqueue method does not exist

D. Missing argument in the last Enqueue call

Solution

1. Step 1: Check method calls

   The last call q.Enqueue(); has no argument, but Enqueue requires one.

2. Step 2: Confirm method signature

   Enqueue needs a value to add, so missing argument causes error.

3. Final Answer:

   Missing argument in the last Enqueue call -> Option D

4. Quick Check:

   Enqueue requires one argument [OK]

Hint: Enqueue always needs one argument to add [OK]

Common Mistakes:

• Calling Enqueue without argument
• Assuming Enqueue can be empty
• Confusing initialization errors

5. You have a queue q with elements [1, 2, 3]. You want to enqueue elements 4 and 5, but only if the queue size is less than 5. Which code snippet correctly performs this conditional enqueue operation in C#?

hard

A. if (q.Count == 5) { q.Enqueue(4); q.Enqueue(5); }

B. if (q.Count <= 5) { q.Enqueue(4); q.Enqueue(5); }

C. if (q.Count < 5) { q.Enqueue(4); q.Enqueue(5); }

D. if (q.Count > 5) { q.Enqueue(4); q.Enqueue(5); }

Solution

1. Step 1: Understand queue size condition

   We want to add only if size is less than 5, so condition is q.Count < 5.

2. Step 2: Apply enqueue calls inside condition

   If condition is true, enqueue 4 and 5 to add them safely.

3. Final Answer:

   if (q.Count < 5) { q.Enqueue(4); q.Enqueue(5); } -> Option C

4. Quick Check:

   Use < 5 to check size before enqueue [OK]

Hint: Check queue size with Count before enqueueing [OK]

Common Mistakes:

• Using <= instead of < causing overflow
• Using > or == which are wrong conditions
• Not checking size before enqueue