Get Length of Linked List in DSA Typescript - Build from Scratch

Get Length of Linked List

📖 Scenario: Imagine you have a chain of boxes connected one after another. Each box holds a number and points to the next box. This is called a linked list. You want to find out how many boxes are in the chain.

🎯 Goal: You will build a simple linked list with three nodes and write code to count how many nodes are in the list.

📋 What You'll Learn

Create a linked list with exactly three nodes containing values 10, 20, and 30

Create a variable called length to count the nodes

Use a while loop to traverse the linked list using a variable called current

Print the length of the linked list

💡 Why This Matters

🌍 Real World

Linked lists are used in many software systems to manage collections of items where the size can change dynamically, like playlists, undo history, or navigation paths.

💼 Career

Understanding linked lists and how to traverse them is a fundamental skill for software developers, especially when working with low-level data structures or preparing for coding interviews.

1

Create a linked list with three nodes

Create a class called Node with two properties: value (number) and next (Node or null). Then create three nodes with values 10, 20, and 30. Link them so that the first node points to the second, and the second points to the third. Finally, create a variable called head that points to the first node.

DSA Typescript

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

// Your code here

Hint

Define the Node class first. Then create three nodes and link them by setting the next property.

2

Create a variable to count the length

Create a variable called length and set it to 0. This will keep track of how many nodes you have counted.

DSA Typescript

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

const node1 = new Node(10);
const node2 = new Node(20);
const node3 = new Node(30);
node1.next = node2;
node2.next = node3;
const head = node1;

// Your code here

Hint

Use let length = 0; to start counting from zero.

3

Traverse the linked list to count nodes

Create a variable called current and set it to head. Use a while loop that runs as long as current is not null. Inside the loop, increase length by 1 and move current to the next node using current = current.next.

DSA Typescript

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

const node1 = new Node(10);
const node2 = new Node(20);
const node3 = new Node(30);
node1.next = node2;
node2.next = node3;
const head = node1;

let length = 0;

// Your code here

Hint

Use a while loop to move through each node until you reach the end (null).

4

Print the length of the linked list

Use console.log(length) to print the number of nodes in the linked list.

DSA Typescript

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

const node1 = new Node(10);
const node2 = new Node(20);
const node3 = new Node(30);
node1.next = node2;
node2.next = node3;
const head = node1;

let length = 0;
let current = head;
while (current !== null) {
  length += 1;
  current = current.next;
}

// Your code here

Hint

Use console.log(length) to show the final count.

Practice

1. What does the function getLength(head: ListNode | null): number typically return when given the head of a linked list?

easy

A. The last node in the linked list

B. The number of nodes in the linked list

C. The value of the first node

D. The sum of all node values

Solution

1. Step 1: Understand the function purpose

   The function named getLength is designed to count nodes, so it returns the number of nodes in the list.

2. Step 2: Confirm what is returned

   Since it counts nodes, it returns the total count, not values or nodes themselves.

3. Final Answer:

   The number of nodes in the linked list -> Option B

4. Quick Check:

   Function name and return type indicate count [OK]

Hint: Function named getLength returns count of nodes [OK]

Common Mistakes:

• Confusing length with node value
• Thinking it returns the last node
• Assuming it sums node values

2. Which of the following TypeScript code snippets correctly initializes a variable count to start counting nodes in a linked list?

easy

A. let count = 0;

B. let count = null;

C. let count = head;

D. let count = undefined;

Solution

1. Step 1: Identify the correct initial value for counting

   Counting nodes requires a number starting at zero, so count should be initialized to 0.

2. Step 2: Check other options

   Initializing to null, head, or undefined is incorrect for counting.

3. Final Answer:

   let count = 0; -> Option A

4. Quick Check:

   Counting starts at zero [OK]

Hint: Start counting nodes from zero [OK]

Common Mistakes:

• Initializing count to null or undefined
• Using head node as count
• Not initializing count at all

3. What will be the output of the following TypeScript code?

class ListNode {
  val: number;
  next: ListNode | null;
  constructor(val: number) {
    this.val = val;
    this.next = null;
  }
}

function getLength(head: ListNode | null): number {
  let count = 0;
  let current = head;
  while (current !== null) {
    count++;
    current = current.next;
  }
  return count;
}

const node1 = new ListNode(10);
const node2 = new ListNode(20);
const node3 = new ListNode(30);
node1.next = node2;
node2.next = node3;

console.log(getLength(node1));

medium

A. 3

B. 2

C. 0

D. Error

Solution

1. Step 1: Trace the linked list nodes

   There are three nodes linked: node1 -> node2 -> node3 -> null.

2. Step 2: Count nodes in the while loop

   The loop increments count for each node until current is null, so count becomes 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   3 nodes counted [OK]

Hint: Count increments once per node until null [OK]

Common Mistakes:

• Counting only 2 nodes by missing last
• Returning zero if loop not entered
• Syntax errors causing runtime error

4. Identify the error in this TypeScript function to get linked list length:

function getLength(head: ListNode | null): number {
  let count = 0;
  let current = head;
  while (current.next !== null) {
    count++;
    current = current.next;
  }
  return count;
}

medium

A. It counts nodes twice

B. It will cause an infinite loop

C. It throws a syntax error

D. It misses counting the last node

Solution

1. Step 1: Analyze the while loop condition

   The loop runs while current.next !== null, so it stops before the last node.

2. Step 2: Understand counting effect

   Because the last node's next is null, it is not counted, so length is one less than actual.

3. Final Answer:

   It misses counting the last node -> Option D

4. Quick Check:

   Loop condition excludes last node [OK]

Hint: Loop until current is null, not current.next [OK]

Common Mistakes:

• Using current.next in loop condition
• Not counting last node
• Assuming loop counts all nodes

5. Given a linked list where each node has a val and next, how would you modify the getLength function to return 0 if the list is empty and the correct length otherwise?

hard

A. Return length as 1 if head is null

B. Start count at 1 and loop while current is not null

C. Check if head is null before loop; return 0 if true, else count nodes

D. Use a for loop from 0 to head.val

Solution

1. Step 1: Handle empty list case

   Check if head is null before counting; if yes, return 0 immediately.

2. Step 2: Count nodes if list is not empty

   If head is not null, loop through nodes incrementing count until current is null.

3. Final Answer:

   Check if head is null before loop; return 0 if true, else count nodes -> Option C

4. Quick Check:

   Empty list returns 0, else counts nodes [OK]

Hint: Return 0 if head is null before counting [OK]

Common Mistakes:

• Starting count at 1 incorrectly
• Returning 1 for empty list
• Using node value instead of node presence