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

Time Complexity: Get Length of Linked List

O(n)

Understanding Time Complexity

We want to understand how the time to find the length of a linked list changes as the list grows.

How does the number of steps increase when the list gets longer?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

function getLength(head: ListNode | null): number {
  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 through each node one by one.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The while loop that visits each node once.
• How many times: Exactly once per node in the list, from start to end.

How Execution Grows With Input

As the list gets longer, the number of steps grows directly with the number of nodes.

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

Pattern observation: The steps increase one by one as the list grows longer.

Final Time Complexity

Time Complexity: O(n)

This means the time to find the length grows in direct proportion to the number of nodes.

Common Mistake

[X] Wrong: "Getting the length is always instant or constant time because we just count nodes."

[OK] Correct: We must visit each node once to count it, so the time depends on how many nodes there are.

Interview Connect

Understanding this helps you explain how simple operations on linked lists scale, a key skill in many coding problems.

Self-Check

"What if the linked list stored its length as a property updated on insertions and deletions? How would the time complexity of getting the length change?"

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