Get Length of Linked List in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a linked list?

A linked list is a chain of nodes where each node holds data and a link to the next node. It is like a treasure hunt where each clue leads to the next one.

beginner

How do you find the length of a linked list?

Start at the first node and move to the next node one by one, counting each node until you reach the end (null). The count is the length.

intermediate

Why can't we use an index to find the length of a linked list like in arrays?

Linked lists do not have indexes because nodes are connected by links, not stored in continuous memory. So, we must count nodes one by one.

intermediate

What is the time complexity of getting the length of a linked list?

It is O(n), where n is the number of nodes, because you must visit each node once to count them.

beginner

Show a simple JavaScript function to get the length of a linked list.

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

What does each node in a linked list contain?

A Data and a link to the next node

B Only data

C Only a link to the next node

D Data and an index

Each node stores data and a link (pointer) to the next node.

What is the first step to find the length of a linked list?

A Start counting from the last node

B Use the index of the nodes

C Start at the head node

D Jump to the middle node

We start counting from the head (first) node and move forward.

What is the time complexity to get the length of a linked list with n nodes?

A O(1)

B O(n)

C O(log n)

D O(n^2)

We must visit each node once, so it takes O(n) time.

Why can't we directly access the 5th node in a linked list?

A Because linked lists have no indexes

B Because nodes are stored in arrays

C Because the 5th node does not exist

D Because nodes are sorted

Linked lists do not have indexes; nodes are connected by links.

What will the function getLength return if the linked list is empty?

A undefined

B 1

C null

D 0

If the list is empty (head is null), length is 0.

Explain step-by-step how to find the length of a linked list.

Think of walking through a chain counting each link.

Write a simple JavaScript function to get the length of a linked list and explain how it works.

Use a while loop and a counter variable.

Practice

1. What does the length of a linked list represent?

easy

A. The memory size of the linked list

B. The value of the first node

C. The number of nodes in the linked list

D. The value of the last node

Solution

1. Step 1: Understand linked list structure

   A linked list is made of nodes connected one after another.

2. Step 2: Define length meaning

   The length counts how many nodes are in the list, regardless of their values.

3. Final Answer:

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

4. Quick Check:

   Length = number of nodes [OK]

Hint: Length counts nodes, not values or memory size [OK]

Common Mistakes:

• Confusing length with node values
• Thinking length is memory size
• Counting only first or last node

2. Which of the following JavaScript code snippets correctly initializes a variable to count the length of a linked list?

easy

A. let length = 0;

B. let length = null;

C. let length = undefined;

D. let length = '0';

Solution

1. Step 1: Identify correct initialization for counting

   Counting starts from zero, so length should be a number 0.

2. Step 2: Check variable types

   Using null, undefined, or string '0' is incorrect for numeric counting.

3. Final Answer:

   let length = 0; -> Option A

4. Quick Check:

   Initialize count with 0 [OK]

Hint: Start counting from zero using a number variable [OK]

Common Mistakes:

• Using null or undefined instead of 0
• Using string '0' instead of number 0
• Not initializing length variable

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

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

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

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

medium

A. 3

B. 2

C. 1

D. 0

Solution

1. Step 1: Trace linked list nodes

   Nodes are 1 -> 2 -> 3 -> null, so 3 nodes total.

2. Step 2: Count nodes in while loop

   Loop increments length for each node until current is null, so length becomes 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Count nodes = 3 [OK]

Hint: Count nodes by traversing until null [OK]

Common Mistakes:

• Stopping loop early
• Not incrementing length
• Incorrectly checking current node

4. Find the error in this code that tries to get the length of a linked list:

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

medium

A. The length variable should start at 1, not 0

B. The loop condition should check current !== null, not current.next !== null

C. The function should return current instead of length

D. The current variable should be initialized to null

Solution

1. Step 1: Analyze loop condition

   Checking current.next !== null misses counting the last node.

2. Step 2: Correct loop condition

   Loop should run while current !== null to count all nodes including last.

3. Final Answer:

   The loop condition should check current !== null, not current.next !== null -> Option B

4. Quick Check:

   Loop until current is null to count all nodes [OK]

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

Common Mistakes:

• Stopping before last node
• Starting length at 1 incorrectly
• Returning wrong variable

5. Given a linked list where each node has a value and a next pointer, how would you modify the length calculation to skip nodes with value 0 and count only nodes with non-zero values?

hard

A. Count all nodes and then subtract the number of zeros found

B. Skip the first node if its value is 0 before starting the loop

C. Set length to the total nodes minus the number of zeros after counting all nodes

D. Increment length only if current.value !== 0 inside the traversal loop

Solution

1. Step 1: Traverse all nodes

   Loop through each node from head to null as usual.

2. Step 2: Count only non-zero value nodes

   Inside loop, increment length only if current.value !== 0 to skip zeros.

3. Final Answer:

   Increment length only if current.value !== 0 inside the traversal loop -> Option D

4. Quick Check:

   Count nodes with value != 0 during traversal [OK]

Hint: Count nodes only if value is not zero during traversal [OK]

Common Mistakes:

• Skipping nodes incorrectly
• Counting all then subtracting zeros
• Modifying list structure