Why Doubly Linked List Over Singly Linked List in DSA Javascript - Quick Recap

Recall & Review

beginner

What is a doubly linked list?

A doubly linked list is a type of linked list where each node has two links: one to the next node and one to the previous node. This allows traversal in both directions.

beginner

Why might a doubly linked list be preferred over a singly linked list?

Because it allows easy backward traversal and efficient deletion of nodes without needing to traverse from the head, unlike singly linked lists.

beginner

What is a key disadvantage of doubly linked lists compared to singly linked lists?

They use more memory because each node stores an extra pointer to the previous node.

intermediate

How does a doubly linked list improve node deletion?

It allows deletion of a node directly by adjusting the previous and next pointers, without needing to find the previous node first.

intermediate

In what scenario is a doubly linked list especially useful?

When you need to traverse the list both forwards and backwards, such as in browser history or undo/redo functionality.

What extra pointer does a doubly linked list node have compared to a singly linked list node?

A Pointer to the head node

B Pointer to the previous node

C Pointer to the tail node

D Pointer to the next-next node

A doubly linked list node has a pointer to the previous node, enabling backward traversal.

Which operation is easier in a doubly linked list than in a singly linked list?

A Backward traversal

B Forward traversal

C Adding a node at the end

D Finding the head node

Backward traversal is easier because each node has a pointer to its previous node.

What is a downside of using a doubly linked list?

A Cannot traverse backwards

B No pointer to next node

C Slower insertion at head

D Uses more memory per node

Doubly linked lists use more memory because each node stores two pointers.

How does a doubly linked list help in deleting a node?

A By adjusting previous and next pointers directly

B By searching from the head every time

C By copying data from the next node

D By using a stack

Deletion is easier because you can access the previous node directly to update pointers.

Which real-life example fits a doubly linked list usage?

A One-way street

B Single direction conveyor belt

C Browser back and forward buttons

D Queue at a ticket counter

Browser history allows moving back and forth, similar to doubly linked list traversal.

Explain why a doubly linked list might be chosen over a singly linked list for certain applications.

Think about how having two pointers per node helps.

Describe the main trade-offs between singly and doubly linked lists.

Consider what each pointer adds or costs.

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