Why Doubly Linked List Over Singly Linked List in DSA Javascript - Performance Analysis

Time Complexity: Why Doubly Linked List Over Singly Linked List

O(n)

Understanding Time Complexity

We want to understand how operations on doubly linked lists compare to singly linked lists in terms of speed.

Which list type lets us do tasks faster and why?

Scenario Under Consideration

Analyze the time complexity of searching and deleting a node in singly vs doubly linked lists.

// Singly Linked List delete operation
function deleteNodeSingly(head, value) {
  if (!head) return null;
  if (head.value === value) return head.next;
  let current = head;
  while (current.next && current.next.value !== value) {
    current = current.next;
  }
  if (current.next) current.next = current.next.next;
  return head;
}

// Doubly Linked List delete operation
function deleteNodeDoubly(head, value) {
  let current = head;
  while (current && current.value !== value) {
    current = current.next;
  }
  if (!current) return head;
  if (current.prev) current.prev.next = current.next;
  if (current.next) current.next.prev = current.prev;
  if (current === head) head = current.next;
  return head;
}
    

These functions remove a node with a given value from singly and doubly linked lists.

Identify Repeating Operations

Both functions search through the list to find the node to delete.

• Primary operation: Traversing nodes one by one to find the target node.
• How many times: Up to n times, where n is the number of nodes.
• In singly linked list, extra step to update previous node's next pointer after finding the node.
• In doubly linked list, direct access to previous node allows immediate pointer updates.

How Execution Grows With Input

As the list grows, the number of steps to find a node grows roughly in a straight line.

Input Size (n)	Approx. Operations
10	Up to 10 steps to find node
100	Up to 100 steps to find node
1000	Up to 1000 steps to find node

Pattern observation: The search time grows linearly with the number of nodes.

Final Time Complexity

Time Complexity: O(n)

This means the time to find and delete a node grows directly with the list size.

Common Mistake

[X] Wrong: "Doubly linked lists always make searching faster than singly linked lists."

[OK] Correct: Both lists require searching from the start, so search time is similar. Doubly linked lists help mainly with easier node deletion once found.

Interview Connect

Understanding these differences helps you explain trade-offs clearly and choose the right list type for your needs.

Self-Check

"What if we kept a tail pointer in a singly linked list? How would that affect the time complexity of deleting the last node?"

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