Get Length of Linked List in DSA Javascript - Time & Space Complexity

Time Complexity: Get Length of Linked List

O(n)

Understanding Time Complexity

We want to know how long it takes to find the length of a linked list as the list grows.

How does the time needed change when the list gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

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

This code counts how many nodes are in a linked list by moving from the start to the end.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The while loop that visits each node once.
• How many times: Once for every node in the list (n times).

How Execution Grows With Input

As the list gets longer, the code checks more nodes one by one.

Input Size (n)	Approx. Operations
10	About 10 steps
100	About 100 steps
1000	About 1000 steps

Pattern observation: The number of steps grows directly with the number of nodes.

Final Time Complexity

Time Complexity: O(n)

This means the time to find the length grows in a straight line with the list size.

Common Mistake

[X] Wrong: "The length can be found instantly without checking nodes."

[OK] Correct: Since linked lists don't store their size, we must visit each node to count them.

Interview Connect

Understanding this helps you explain how linked lists work and how to measure their size efficiently.

Self-Check

"What if the linked list stored its length as a property? How would the time complexity change?"

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