Why Doubly Linked List Over Singly Linked List in DSA Javascript - See It Work

Why Doubly Linked List Over Singly Linked List

📖 Scenario: Imagine you are managing a playlist of songs on a music app. You want to move forward and backward through songs easily.

🎯 Goal: You will build a simple doubly linked list to understand why it is better than a singly linked list for moving both forward and backward.

📋 What You'll Learn

Create a doubly linked list with three nodes containing song names

Add a variable to track the current song node

Write code to move forward to the next song

Write code to move backward to the previous song

Print the current song after moving forward and backward

💡 Why This Matters

🌍 Real World

Music apps, photo galleries, and browsers use doubly linked lists to let users move forward and backward smoothly.

💼 Career

Understanding doubly linked lists helps in software development roles where efficient navigation and editing of data sequences are needed.

1

Create a doubly linked list with three songs

Create three nodes called node1, node2, and node3. Each node should be an object with a song property set to 'Song A', 'Song B', and 'Song C' respectively. Also, add next and prev properties to link the nodes in order.

DSA Javascript

// Create node1, node2, node3 with song names and link them using next and prev
// Your code here

Hint

Each node is an object with song, next, and prev properties. Link them so node1.next is node2, and so on.

2

Set the current song to the first node

Create a variable called currentSong and set it to node1 to start at the first song.

DSA Javascript

const node1 = { song: 'Song A', next: null, prev: null };
const node2 = { song: 'Song B', next: null, prev: null };
const node3 = { song: 'Song C', next: null, prev: null };

node1.next = node2;
node2.prev = node1;
node2.next = node3;
node3.prev = node2;

// Set currentSong to node1
// Your code here

Hint

Just assign node1 to a new variable currentSong.

3

Move forward to the next song

Change currentSong to point to the next song using the next property.

DSA Javascript

const node1 = { song: 'Song A', next: null, prev: null };
const node2 = { song: 'Song B', next: null, prev: null };
const node3 = { song: 'Song C', next: null, prev: null };

node1.next = node2;
node2.prev = node1;
node2.next = node3;
node3.prev = node2;

let currentSong = node1;

// Move currentSong to the next song
// Your code here

Hint

Use currentSong.next to move forward.

4

Move backward to the previous song and print current song

Change currentSong to point to the previous song using the prev property. Then print the current song name using console.log(currentSong.song).

DSA Javascript

const node1 = { song: 'Song A', next: null, prev: null };
const node2 = { song: 'Song B', next: null, prev: null };
const node3 = { song: 'Song C', next: null, prev: null };

node1.next = node2;
node2.prev = node1;
node2.next = node3;
node3.prev = node2;

let currentSong = node1;

currentSong = currentSong.next;

// Move currentSong to the previous song
// Print the current song name
// Your code here

Hint

Use currentSong.prev to move backward. Then print the song name with console.log(currentSong.song).

Practice

1. Why might you choose a doubly linked list over a singly linked list in a program?

easy

A. Because it only stores one pointer per node

B. Because it uses less memory than singly linked list

C. Because it allows traversal in both directions easily

D. Because it is simpler to implement

Solution

1. Step 1: Understand traversal directions

   A singly linked list allows moving only forward through nodes, while a doubly linked list allows moving forward and backward.

2. Step 2: Compare memory and complexity

   Doubly linked lists use more memory because each node stores two pointers, but they enable easier backward traversal.

3. Final Answer:

   Because it allows traversal in both directions easily -> Option C

4. Quick Check:

   Traversal directions = Both ways [OK]

Hint: Doubly linked lists have two pointers per node [OK]

Common Mistakes:

• Thinking doubly linked lists use less memory
• Confusing simplicity with singly linked lists
• Assuming singly linked lists allow backward traversal

2. Which of the following is the correct way to define a node in a doubly linked list in JavaScript?

easy

A. class Node { constructor(value) { this.value = value; this.prev = null; this.next = null; } }

B. class Node { constructor(value) { this.value = value; this.prev = null; } }

C. class Node { constructor(value) { this.value = value; this.next = null; } }

D. class Node { constructor(value) { this.value = value; this.next = null; this.prev = value; } }

Solution

1. Step 1: Identify required pointers in doubly linked list node

   A doubly linked list node must have both prev and next pointers to link backward and forward.

2. Step 2: Check constructor correctness

   class Node { constructor(value) { this.value = value; this.prev = null; this.next = null; } } correctly initializes prev and next to null along with the value.

3. Final Answer:

   class Node { constructor(value) { this.value = value; this.prev = null; this.next = null; } } -> Option A

4. Quick Check:

   Node has prev and next pointers [OK]

Hint: Doubly linked list nodes have prev and next pointers [OK]

Common Mistakes:

• Omitting the prev pointer
• Assigning prev to value instead of null
• Defining only next pointer

3. Consider this code snippet for a doubly linked list traversal backward:

let current = tail;
let result = [];
while (current !== null) {
  result.push(current.value);
  current = current.prev;
}
console.log(result);

What will be the output if the list contains nodes with values 1 -> 2 -> 3 (head to tail)?

medium

A. [3, 2, 1]

B. [1, 2, 3]

C. [null]

D. Error: current.prev is undefined

Solution

1. Step 1: Understand traversal starting point

   The traversal starts at tail, which holds the value 3 in the list 1 -> 2 -> 3.

2. Step 2: Trace backward traversal

   It pushes values 3, then moves to 2, pushes 2, then moves to 1, pushes 1, then stops at null.

3. Final Answer:

   [3, 2, 1] -> Option A

4. Quick Check:

   Backward traversal outputs reversed list [OK]

Hint: Backward traversal prints list in reverse order [OK]

Common Mistakes:

• Assuming output is forward order
• Confusing head and tail starting points
• Expecting error due to null check missing

4. The following code tries to remove the last node from a doubly linked list but causes an error:

function removeLast(tail) {
  if (!tail) return null;
  let prevNode = tail.prev;
  prevNode.next = null;
  return prevNode;
}

What is the error and how to fix it?

medium

A. Error because tail is not updated; fix by returning tail instead

B. Error if tail.prev is null; fix by checking if prevNode exists before setting next

C. No error; code works fine

D. Error because next pointer should be set to tail, fix by assigning tail.next = null

Solution

1. Step 1: Identify error cause

   If the list has only one node, tail.prev is null, so prevNode.next causes an error.

2. Step 2: Fix by adding null check

   Before setting prevNode.next = null, check if prevNode exists to avoid error.

3. Final Answer:

   Error if tail.prev is null; fix by checking if prevNode exists before setting next -> Option B

4. Quick Check:

   Check prevNode before access [OK]

Hint: Check for null before accessing prevNode.next [OK]

Common Mistakes:

• Ignoring single-node list edge case
• Assuming tail always has a prev node
• Not returning updated tail correctly

5. You want to implement a feature that requires frequent backward and forward traversal and quick deletion of nodes from anywhere in the list. Which data structure is best and why?

hard

A. Stack, because it supports LIFO operations

B. Singly linked list, because it uses less memory

C. Array, because it allows random access

D. Doubly linked list, because it supports traversal in both directions and easy deletion

Solution

1. Step 1: Analyze traversal needs

   Frequent backward and forward traversal requires a structure that supports moving in both directions easily.

2. Step 2: Analyze deletion needs

   Quick deletion from anywhere is easier if nodes have pointers to previous nodes, so you can update links without full traversal.

3. Step 3: Compare options

   Singly linked lists lack backward pointers, arrays have costly deletions, stacks only allow LIFO access.

4. Final Answer:

   Doubly linked list, because it supports traversal in both directions and easy deletion -> Option D

5. Quick Check:

   Bidirectional traversal + easy deletion = Doubly linked list [OK]

Hint: Bidirectional traversal needs doubly linked list [OK]

Common Mistakes:

• Choosing singly linked list for backward traversal
• Choosing array ignoring deletion cost
• Confusing stack with linked list