Enqueue Using Linked List in DSA Typescript - Build from Scratch

Enqueue Using Linked List

📖 Scenario: Imagine you are managing a line of customers waiting to buy tickets. You want to add new customers to the end of the line efficiently. This line is represented using a linked list.

🎯 Goal: Build a simple linked list queue and add new elements to the end using the enqueue operation.

📋 What You'll Learn

Create a Node class with value and next properties

Create a Queue class with head and tail properties

Implement an enqueue method to add nodes at the end

Print the queue values from head to tail after enqueue operations

💡 Why This Matters

🌍 Real World

Queues are used in real life to manage lines, like customers waiting for service or tasks waiting to be processed.

💼 Career

Understanding how to implement queues with linked lists helps in software development roles that require efficient data handling and memory management.

1

Create Node and Queue classes

Create a class called Node with a constructor that takes a value of type number and sets this.value and this.next to null. Then create a class called Queue with head and tail properties both initialized to null.

DSA Typescript

// Your code here to create Node and Queue classes

Hint

Think of Node as one person in the line and Queue as the whole line.

2

Add enqueue method to Queue

Inside the Queue class, add a method called enqueue that takes a value of type number. Create a new Node with this value. If tail is not null, set tail.next to the new node. Then update tail to the new node. If head is null, set head to the new node.

DSA Typescript

class Node {
    value: number;
    next: Node | null;
    constructor(value: number) {
        this.value = value;
        this.next = null;
    }
}

class Queue {
    head: Node | null;
    tail: Node | null;
    constructor() {
        this.head = null;
        this.tail = null;
    }

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

Hint

Remember to update both head and tail when the queue is empty.

3

Enqueue values 10, 20, and 30

Create a new Queue instance called queue. Use the enqueue method to add the values 10, 20, and 30 to the queue.

DSA Typescript

class Node {
    value: number;
    next: Node | null;
    constructor(value: number) {
        this.value = value;
        this.next = null;
    }
}

class Queue {
    head: Node | null;
    tail: Node | null;
    constructor() {
        this.head = null;
        this.tail = null;
    }

    enqueue(value: number): void {
        const newNode = new Node(value);
        if (this.tail) {
            this.tail.next = newNode;
        }
        this.tail = newNode;
        if (!this.head) {
            this.head = newNode;
        }
    }
}

// Create queue and enqueue values
// Your code here

Hint

Use the enqueue method three times with the exact values.

4

Print the queue values from head to tail

Write a printQueue function that takes a Queue instance and prints the values from head to tail separated by -> and ending with null. Then call printQueue(queue) to display the queue contents.

DSA Typescript

class Node {
    value: number;
    next: Node | null;
    constructor(value: number) {
        this.value = value;
        this.next = null;
    }
}

class Queue {
    head: Node | null;
    tail: Node | null;
    constructor() {
        this.head = null;
        this.tail = null;
    }

    enqueue(value: number): void {
        const newNode = new Node(value);
        if (this.tail) {
            this.tail.next = newNode;
        }
        this.tail = newNode;
        if (!this.head) {
            this.head = newNode;
        }
    }
}

const queue = new Queue();
queue.enqueue(10);
queue.enqueue(20);
queue.enqueue(30);

// Write a function printQueue(queue: Queue): void to print values
// Then call printQueue(queue)
// Your code here

Hint

Use a loop to visit each node starting from head and build a string to print.

Practice

1. What does the enqueue operation do in a linked list implementation of a queue?

easy

A. Removes the first element from the linked list

B. Adds a new element at the end (tail) of the linked list

C. Adds a new element at the beginning (head) of the linked list

D. Deletes the last element from the linked list

Solution

1. Step 1: Understand enqueue operation

   Enqueue means adding an element to the queue.

2. Step 2: Identify linked list position for enqueue

   In a linked list queue, new elements are added at the tail (end).

3. Final Answer:

   Adds a new element at the end (tail) of the linked list -> Option B

4. Quick Check:

   Enqueue = Add at tail [OK]

Hint: Enqueue always adds at the tail in linked list queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue operation
• Adding new element at head instead of tail
• Thinking enqueue removes elements
• Mixing linked list positions

2. Which of the following is the correct TypeScript syntax to create a new node for enqueue in a linked list?

easy

A. newNode = new Node();

B. let newNode = Node(value);

C. const newNode = new Node(value);

D. const newNode = Node.new(value);

Solution

1. Step 1: Recall TypeScript class instantiation

   To create an object, use new ClassName(params).

2. Step 2: Check options for correct syntax

   Only const newNode = new Node(value); correctly creates a new Node instance.

3. Final Answer:

   const newNode = new Node(value); -> Option C

4. Quick Check:

   Use 'new' keyword to instantiate class [OK]

Hint: Use 'new' keyword to create class instances in TypeScript [OK]

Common Mistakes:

• Omitting 'new' keyword when creating objects
• Using function call syntax without 'new'
• Incorrect property access like Node.new
• Not declaring variable with let or const

3. What will be the state of the queue after executing the following enqueue operations on an empty linked list queue?

enqueue(10); enqueue(20); enqueue(30);

medium

A. 10 -> 20 -> 30 -> null

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

C. null -> 10 -> 20 -> 30

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

Solution

1. Step 1: Understand enqueue order

   Enqueue adds elements at the tail, so order is preserved as inserted.

2. Step 2: Trace each enqueue

   After enqueue(10): 10 -> null
   After enqueue(20): 10 -> 20 -> null
   After enqueue(30): 10 -> 20 -> 30 -> null

3. Final Answer:

   10 -> 20 -> 30 -> null -> Option A

4. Quick Check:

   Enqueue order = insertion order [OK]

Hint: Enqueue adds at tail, so order stays same as insertion [OK]

Common Mistakes:

• Reversing order of elements after enqueue
• Confusing enqueue with push at head
• Misreading linked list direction
• Ignoring null at the end

4. Consider this enqueue function snippet for a linked list queue:

enqueue(value: number) {
  const newNode = new Node(value);
  if (!this.tail) {
    this.head = newNode;
    this.tail = newNode;
    return;
  }
  this.tail.next = newNode;
  this.tail = newNode;
}

What is the bug in this code?

medium

A. It does not update the previous tail's next pointer to newNode

B. It incorrectly sets head to null when queue is empty

C. It creates newNode without value

D. It sets tail to null after enqueue

Solution

1. Step 1: Analyze enqueue logic

   When queue is empty, head and tail should point to newNode.

2. Step 2: Check linking of nodes

   Code sets tail = newNode but does not link old tail's next to newNode, breaking chain.

3. Final Answer:

   It does not update the previous tail's next pointer to newNode -> Option A

4. Quick Check:

   Link old tail's next to newNode [OK]

Hint: Always link old tail's next to new node when enqueueing [OK]

Common Mistakes:

• Forgetting to update old tail's next pointer
• Not handling empty queue case properly
• Setting tail to null accidentally
• Creating node without value parameter

5. You have a linked list queue with elements: 5 -> 15 -> 25 -> null.
After enqueue(35), what will be the correct linked list representation and tail node value?

hard

A. 5 -> 15 -> 25 -> 35 -> null, tail value = 25

B. 35 -> 5 -> 15 -> 25 -> null, tail value = 25

C. 5 -> 15 -> 25 -> null, tail value = 25

D. 5 -> 15 -> 25 -> 35 -> null, tail value = 35

Solution

1. Step 1: Understand enqueue effect on queue

   Enqueue adds new element at tail, so 35 goes after 25.

2. Step 2: Update tail pointer

   Tail must point to the new last node, which holds 35.

3. Final Answer:

   5 -> 15 -> 25 -> 35 -> null, tail value = 35 -> Option D

4. Quick Check:

   Enqueue adds at tail, tail updates to new node [OK]

Hint: Tail always points to last enqueued node [OK]

Common Mistakes:

• Adding new node at head instead of tail
• Not updating tail pointer after enqueue
• Confusing tail value with head value
• Leaving tail pointing to old last node