Why Doubly Linked List Over Singly Linked List in DSA Javascript - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a new node at the end of a singly linked list.

DSA Javascript

function appendNode(head, value) {
  let newNode = { data: value, next: null };
  if (!head) return newNode;
  let current = head;
  while (current.[1] !== null) {
    current = current.next;
  }
  current.next = newNode;
  return head;
}

Drag options to blanks, or click blank then click option'

Anext

Bprev

Cdata

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' instead of 'next' to traverse the list.

Trying to access 'data' to move to the next node.

In a singly linked list, each node points to the next node using the 'next' property. To traverse to the end, we check 'current.next'.

2fill in blank

medium

Complete the code to insert a new node before a given node in a doubly linked list.

DSA Javascript

function insertBefore(node, value) {
  let newNode = { data: value, prev: null, next: null };
  newNode.next = node;
  newNode.[1] = node.prev;
  if (node.prev) {
    node.prev.next = newNode;
  }
  node.prev = newNode;
  return newNode;
}

Drag options to blanks, or click blank then click option'

Aprev

Bnext

Cdata

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Setting newNode.next instead of newNode.prev.

Not updating node.prev to newNode.

In a doubly linked list, each node has 'prev' and 'next'. To insert before, set newNode.prev to node.prev.

3fill in blank

hard

Fix the error in the code to correctly remove a node from a doubly linked list.

DSA Javascript

function removeNode(node) {
  if (node.prev) {
    node.prev.next = node.[1];
  }
  if (node.next) {
    node.next.prev = node.prev;
  }
  node.prev = null;
  node.next = null;
}

Drag options to blanks, or click blank then click option'

Aprev

Bdata

Cnext

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'prev' instead of 'next' when updating node.prev.next.

Not clearing node.prev and node.next after removal.

To remove a node, link its previous node's 'next' to its next node.

4fill in blank

hard

Fill both blanks to create a function that traverses a doubly linked list backwards from tail to head.

DSA Javascript

function traverseBackward(tail) {
  let current = tail;
  while (current !== null) {
    console.log(current.data);
    current = current.[1];
  }
}

// To start from tail, find tail by moving from head
function findTail(head) {
  let current = head;
  while (current.[2] !== null) {
    current = current.next;
  }
  return current;
}

Drag options to blanks, or click blank then click option'

Aprev

Bnext

Cdata

Dhead

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' to traverse backward.

Using 'prev' to find tail from head.

To move backward, use 'prev'. To find tail from head, move forward using 'next'.

5fill in blank

hard

Fill all three blanks to create a function that converts a singly linked list to a doubly linked list.

DSA Javascript

function convertToDoubly(head) {
  let current = head;
  let prev = null;
  while (current !== null) {
    current.[1] = prev;
    let nextNode = current.[2];
    current.[3] = nextNode;
    prev = current;
    current = nextNode;
  }
  return head;
}

Drag options to blanks, or click blank then click option'

Aprev

Bnext

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing 'prev' and 'next' pointers.

Not updating 'prev' variable inside the loop.

To convert, add 'prev' pointer to current node, keep 'next' pointer intact.

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