Linked List vs Array When to Choose Which in DSA Typescript - Complexity Comparison

Time Complexity: Linked List vs Array When to Choose Which

O(1) for array access, O(n) for linked list access

Understanding Time Complexity

We want to understand how fast operations happen in arrays and linked lists.

This helps us decide which one to use for different tasks.

Scenario Under Consideration

Analyze the time complexity of accessing and inserting elements in arrays and linked lists.

// Access element at index i in array
const element = array[i];

// Insert element at start of linked list
const newNode = { value: x, next: head };
head = newNode;
    

This code shows accessing an element in an array and inserting at the start of a linked list.

Identify Repeating Operations

Look at what repeats when we do these operations many times.

• Primary operation: Access by index in array, and traversal in linked list for access.
• How many times: Access in array is direct (once), linked list access requires walking nodes one by one.

How Execution Grows With Input

As the list or array grows, accessing elements behaves differently.

Input Size (n)	Array Access Steps	Linked List Access Steps
10	1	Up to 10
100	1	Up to 100
1000	1	Up to 1000

Access in array stays fast no matter size; linked list access grows linearly with size.

Final Time Complexity

Time Complexity: O(1) for array access, O(n) for linked list access

This means arrays let you get items instantly, but linked lists take longer as they grow.

Common Mistake

[X] Wrong: "Linked lists are always slower than arrays for all operations."

[OK] Correct: Linked lists are faster for adding or removing items at the start because they don't need to shift elements like arrays do.

Interview Connect

Knowing when to use arrays or linked lists shows you understand how data structures affect speed.

Self-Check

"What if we needed to insert elements only at the end of the list? How would the time complexity change for arrays and linked lists?"

Practice

1. Which data structure is better when you need fast random access to elements by index?

easy

A. Stack

B. Linked List

C. Array

D. Queue

Solution

1. Step 1: Understand random access in arrays

   Arrays store elements in contiguous memory, so accessing by index is fast and direct.

2. Step 2: Understand random access in linked lists

   Linked lists require traversal from the head to reach an index, which is slower.

3. Final Answer:

   Array -> Option C

4. Quick Check:

   Fast random access = Array [OK]

Hint: Arrays give fast index access; linked lists do not [OK]

Common Mistakes:

• Thinking linked lists allow fast index access
• Confusing stack or queue with arrays
• Assuming all data structures have same access speed

2. Which of the following is the correct way to declare an empty linked list node in TypeScript?

easy

A. const node = new LinkedListNode(null);

B. const node = new LinkedListNode();

C. const node = LinkedListNode();

D. const node = LinkedListNode(null);

Solution

1. Step 1: Understand TypeScript class instantiation

   To create an instance, use the 'new' keyword with constructor parameters.

2. Step 2: Check constructor parameters

   LinkedListNode usually requires a value; null can represent empty data.

3. Final Answer:

   const node = new LinkedListNode(null); -> Option A

4. Quick Check:

   Use 'new' with constructor args for instances [OK]

Hint: Use 'new' keyword with constructor parameters to create nodes [OK]

Common Mistakes:

• Omitting 'new' keyword
• Calling class like a function without 'new'
• Passing no arguments when constructor expects one

3. Given this TypeScript code, what is the printed output?

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

const head = new Node(1);
head.next = new Node(2);
head.next.next = new Node(3);

let current = head;
const result: number[] = [];
while (current !== null) {
  result.push(current.value);
  current = current.next;
}
console.log(result);

medium

A. [1, 2, 3]

B. [3, 2, 1]

C. [1, 3, 2]

D. []

Solution

1. Step 1: Build the linked list nodes

   Nodes created with values 1, 2, 3 linked in order: 1 -> 2 -> 3 -> null.

2. Step 2: Traverse and collect values

   Loop pushes values 1, then 2, then 3 into result array.

3. Final Answer:

   [1, 2, 3] -> Option A

4. Quick Check:

   Traverse linked list in order = [1, 2, 3] [OK]

Hint: Traverse linked list from head to null to get values in order [OK]

Common Mistakes:

• Confusing order of nodes
• Not updating current pointer correctly
• Expecting reversed output

4. This TypeScript code tries to insert a new node at the start of a linked list but has a bug. What is the error?

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

let head = new Node(1);
head.next = new Node(2);

function insertAtStart(newValue: number) {
  const newNode = new Node(newValue);
  newNode.next = head.next;
  head = newNode;
}

insertAtStart(0);

let current = head;
const result: number[] = [];
while (current !== null) {
  result.push(current.value);
  current = current.next;
}
console.log(result);

medium

A. head should not be reassigned inside the function

B. newNode.next should point to head, not head.next

C. Node constructor missing initialization of next

D. insertAtStart should return the new head

Solution

1. Step 1: Analyze insertion logic

   New node's next is set to head.next, skipping the current head node.

2. Step 2: Correct pointer assignment

   To insert at start, newNode.next must point to current head, preserving the list.

3. Final Answer:

   newNode.next should point to head, not head.next -> Option B

4. Quick Check:

   Insert at start = newNode.next = head [OK]

Hint: New head's next must link to old head, not old head's next [OK]

Common Mistakes:

• Setting newNode.next to head.next causes node loss
• Not updating head reference properly
• Confusing next pointers in linked list

5. You need to frequently insert and delete elements in the middle of a large collection. Which data structure is better and why? A) Array, because it allows fast random access. B) Linked List, because it supports binary search efficiently. C) Array, because it uses less memory. D) Linked List, because it allows fast insertion and deletion without shifting elements.

hard

A. Array, because it allows fast random access.

B. Linked List, because it supports binary search efficiently.

C. Array, because it uses less memory.

D. Linked List, because it allows fast insertion and deletion without shifting elements.

Solution

1. Step 1: Consider insertion and deletion in arrays

   Arrays require shifting elements when inserting or deleting in the middle, which is slow.

2. Step 2: Consider linked list advantages

   Linked lists can insert or delete nodes by changing pointers without shifting elements, making these operations faster.

3. Final Answer:

   Linked List, because it allows fast insertion and deletion without shifting elements. -> Option D

4. Quick Check:

   Frequent middle insert/delete = Linked List [OK]

Hint: Linked lists excel at middle insert/delete without shifting [OK]

Common Mistakes:

• Choosing array for frequent middle insert/delete
• Thinking linked lists support binary search efficiently
• Ignoring cost of shifting elements in arrays