Queue Implementation Using Linked List in DSA Java - Build from Scratch

Queue Implementation Using Linked List

📖 Scenario: You are building a simple queue system for a ticket counter. People join the queue and wait for their turn. You will use a linked list to manage the queue efficiently.

🎯 Goal: Create a queue using a linked list with basic operations: enqueue (add to queue), dequeue (remove from queue), and display the queue.

📋 What You'll Learn

Create a Node class to represent each person in the queue with an integer data and a next pointer.

Create a Queue class with front and rear pointers initialized to null.

Implement enqueue(int data) method to add a new node at the rear of the queue.

Implement dequeue() method to remove a node from the front of the queue and return its data.

Implement display() method to print the queue elements from front to rear.

Demonstrate the queue by enqueueing three elements, dequeueing one, then displaying the queue.

💡 Why This Matters

🌍 Real World

Queues are used in real life for managing lines, like in banks, ticket counters, and call centers.

💼 Career

Understanding queue implementation helps in software development roles involving task scheduling, resource management, and real-time processing.

1

Create Node class and Queue class with front and rear

Create a Node class with an integer data and a Node next pointer. Then create a Queue class with Node front and Node rear variables initialized to null.

DSA Java

// Define Node class
// Define Queue class with front and rear initialized to null
// Your code here

Hint

Think of Node as a box holding a number and a pointer to the next box. The Queue keeps track of the first and last boxes.

2

Add enqueue method to add elements at the rear

In the Queue class, add a method void enqueue(int data) that creates a new Node with the given data and adds it at the rear of the queue. Update front and rear pointers accordingly.

DSA Java

class Node {
    int data;
    Node next;

    Node(int data) {
        this.data = data;
        this.next = null;
    }
}

class Queue {
    Node front = null;
    Node rear = null;

    // Add enqueue method here
    // Your code here
}

Hint

If the queue is empty, both front and rear point to the new node. Otherwise, link the new node at the end and move rear.

3

Add dequeue method to remove elements from the front

In the Queue class, add a method int dequeue() that removes the node at the front of the queue and returns its data. Update front and rear pointers accordingly. If the queue is empty, return -1.

DSA Java

class Node {
    int data;
    Node next;

    Node(int data) {
        this.data = data;
        this.next = null;
    }
}

class Queue {
    Node front = null;
    Node rear = null;

    void enqueue(int data) {
        Node newNode = new Node(data);
        if (rear == null) {
            front = newNode;
            rear = newNode;
        } else {
            rear.next = newNode;
            rear = newNode;
        }
    }

    // Add dequeue method here
    // Your code here
}

Hint

Remove the first node by moving front to the next node. If queue becomes empty, set rear to null.

4

Add display method and demonstrate queue operations

In the Queue class, add a method void display() that prints the queue elements from front to rear separated by ->. Then, in the main method, create a Queue object, enqueue 10, 20, 30, dequeue one element, and display the queue.

DSA Java

class Node {
    int data;
    Node next;

    Node(int data) {
        this.data = data;
        this.next = null;
    }
}

class Queue {
    Node front = null;
    Node rear = null;

    void enqueue(int data) {
        Node newNode = new Node(data);
        if (rear == null) {
            front = newNode;
            rear = newNode;
        } else {
            rear.next = newNode;
            rear = newNode;
        }
    }

    int dequeue() {
        if (front == null) {
            return -1;
        }
        int value = front.data;
        front = front.next;
        if (front == null) {
            rear = null;
        }
        return value;
    }

    // Add display method here
    // Your code here
}

public class Main {
    public static void main(String[] args) {
        // Create Queue object and test enqueue, dequeue, display
        // Your code here
    }
}

Hint

Traverse from front to rear printing each node's data. In main, add three numbers, remove one, then print the queue.

Practice

1. What is the main advantage of implementing a queue using a linked list over an array?

easy

A. Dynamic size, no fixed limit on queue length

B. Faster access to elements by index

C. Uses less memory than arrays

D. Simpler to implement than arrays

Solution

1. Step 1: Understand queue size limitation in arrays

   Arrays have fixed size, so queues implemented with arrays have a maximum capacity.

2. Step 2: Linked list allows dynamic size

   Linked lists can grow or shrink as needed, so queues using linked lists have no fixed size limit.

3. Final Answer:

   Dynamic size, no fixed limit on queue length -> Option A

4. Quick Check:

   Linked list queue size = dynamic [OK]

Hint: Linked list queues grow dynamically, arrays do not [OK]

Common Mistakes:

• Thinking arrays always use less memory
• Confusing access speed with size flexibility
• Assuming linked lists are simpler to implement

2. Which of the following is the correct way to declare a node class for a linked list queue in Java?

easy

A. class Node { int data; int next; }

B. class Node { int data; Node prev; }

C. class Node { Node data; Node next; }

D. class Node { int data; Node next; }

Solution

1. Step 1: Identify node fields for linked list queue

   A node should store data and a reference to the next node.

2. Step 2: Check each option's correctness

   class Node { int data; Node next; } has int data and Node next, which is correct. Others have incorrect types or missing next pointer.

3. Final Answer:

   class Node { int data; Node next; } -> Option D

4. Quick Check:

   Node stores data + next pointer [OK]

Hint: Node must link to next node, not previous or int [OK]

Common Mistakes:

• Using 'prev' pointer instead of 'next' for queue node
• Declaring next as int instead of Node
• Using Node type for data field

3. What will be the output after executing the following code snippet for a linked list queue?

Queue q = new Queue();
q.enqueue(10);
q.enqueue(20);
q.dequeue();
q.enqueue(30);
q.display();

Assuming display() prints elements front to rear separated by ' -> ' and ends with ' -> null'.

medium

A. 20 -> 30 -> null

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

C. 30 -> null

D. 20 -> null

Solution

1. Step 1: Trace enqueue and dequeue operations

   Enqueue 10: queue = 10
   Enqueue 20: queue = 10 -> 20
   Dequeue: removes 10, queue = 20
   Enqueue 30: queue = 20 -> 30

2. Step 2: Display queue elements

   Queue elements front to rear are 20 followed by 30, so output is '20 -> 30 -> null'.

3. Final Answer:

   20 -> 30 -> null -> Option A

4. Quick Check:

   After dequeue, front is 20, then 30 [OK]

Hint: Dequeue removes front; enqueue adds rear [OK]

Common Mistakes:

• Not removing the first element on dequeue
• Appending new elements at front instead of rear
• Confusing order of elements in display

4. Identify the error in this dequeue method of a linked list queue:

public int dequeue() {
  if (front == null) {
    System.out.println("Queue is empty");
    return -1;
  }
  int value = front.data;
  front = front.next;
  if (front == null) {
    rear = null;
  }
  return value;
}

medium

A. Should update rear before front

B. No error, method works correctly

C. Returns wrong value after dequeue

D. Does not handle empty queue properly

Solution

1. Step 1: Check empty queue handling

   Method checks if front is null and returns -1 with message, which is correct.

2. Step 2: Check updating front and rear pointers

   After removing front node, front is updated to next node. If front becomes null, rear is also set to null. This is correct to keep queue consistent.

3. Final Answer:

   No error, method works correctly -> Option B

4. Quick Check:

   Dequeue updates front and rear properly [OK]

Hint: Set rear null only if front becomes null after dequeue [OK]

Common Mistakes:

• Not setting rear to null when queue becomes empty
• Returning wrong value after dequeue
• Not checking for empty queue before dequeue

5. You want to implement a method reverseQueue() that reverses the order of elements in a linked list queue without using extra data structures. Which approach is best?

hard

A. Create a new linked list and enqueue elements in reverse order

B. Traverse linked list and swap data of front and rear nodes repeatedly

C. Use recursion to dequeue elements and enqueue them back in reverse order

D. Use a stack to store elements and then enqueue back

Solution

1. Step 1: Understand constraints

   We cannot use extra data structures like stack or new linked list, so options C and D are invalid.

2. Step 2: Evaluate remaining options

   Traverse linked list and swap data of front and rear nodes repeatedly swaps data repeatedly but is inefficient and complex. Use recursion to dequeue elements and enqueue them back in reverse order uses recursion to dequeue all elements, then enqueue them back in reverse order, effectively reversing the queue without extra structures.

3. Final Answer:

   Use recursion to dequeue elements and enqueue them back in reverse order -> Option C

4. Quick Check:

   Recursion reverses queue without extra structures [OK]

Hint: Recursion can reverse queue without extra storage [OK]

Common Mistakes:

• Using extra data structures despite constraints
• Trying to swap node data instead of rearranging nodes
• Not handling empty or single-element queue cases