Get Length of Linked List in DSA Python - 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.

class Node:
    def __init__(self, value):
        self.value = value
        self.next = None

def get_length(head):
    count = 0
    current = head
    while current is not None:
        count += 1
        current = current.next
    return count

This code counts how many nodes are in a linked list by moving from the first node to the last.

Identify Repeating Operations

• Primary operation: The while loop that visits each node once.
• How many times: Exactly once per node in the list, so as many times as the list length.

How Execution Grows With Input

Each new node adds one more step to count it, so the steps grow directly with the list size.

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

Pattern observation: The number of steps grows in a straight line with the list size.

Final Time Complexity

Time Complexity: O(n)

This means the time to find the length grows directly with how many nodes are in the list.

Common Mistake

[X] Wrong: "The length can be found instantly without looking at each node."

[OK] Correct: Because a linked list does not store its size, you must check each node to count them.

Interview Connect

Knowing how to analyze simple linked list operations like this builds a strong base for more complex problems.

Self-Check

"What if the linked list stored its length as a property updated on insertions? 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 is how many nodes are present in the list.

3. Final Answer:

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

4. Quick Check:

   Length counts nodes = number of nodes [OK]

Hint: Length = count of nodes from head to end [OK]

Common Mistakes:

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

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

easy

A. length = None

B. length = 0

C. length = head

D. length = []

Solution

1. Step 1: Identify counting variable type

   Length should be a number starting at zero before counting nodes.

2. Step 2: Check each option

   length = 0 sets length to 0, ready to increment. Others assign wrong types.

3. Final Answer:

   length = 0 -> Option B

4. Quick Check:

   Initialize count as 0 before loop [OK]

Hint: Start counting length from zero [OK]

Common Mistakes:

• Initializing length as None or list
• Assigning length to head node
• Using wrong data types for count

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

class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

def get_length(head):
    count = 0
    current = head
    while current:
        count += 1
        current = current.next
    return count

# Creating linked list: 10 -> 20 -> 30 -> None
head = Node(10)
head.next = Node(20)
head.next.next = Node(30)

print(get_length(head))

medium

A. 3

B. 2

C. 0

D. None

Solution

1. Step 1: Trace linked list nodes

   List has nodes with values 10, 20, and 30, total 3 nodes.

2. Step 2: Follow get_length function

   Function counts nodes by moving current pointer until None, increments count each step.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Count nodes 10,20,30 = 3 [OK]

Hint: Count nodes by traversing until None [OK]

Common Mistakes:

• Stopping count early
• Not incrementing count inside loop
• Returning None instead of count

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

def get_length(head):
    count = 0
    while head.next:
        count += 1
        head = head.next
    return count

medium

A. It misses counting the last node

B. It causes infinite loop

C. It returns None instead of count

D. It counts nodes twice

Solution

1. Step 1: Analyze loop condition

   Loop runs while head.next exists, so it stops before last node.

2. Step 2: Check counting logic

   Last node is not counted because loop ends early, count misses one node.

3. Final Answer:

   It misses counting the last node -> Option A

4. Quick Check:

   Loop condition excludes last node [OK]

Hint: Use 'while current' not 'while current.next' to count all nodes [OK]

Common Mistakes:

• Using head.next in loop condition
• Not counting last node
• Assuming loop covers all nodes

5. You have a linked list where each node contains a number. Write a function to find the length of the linked list only if all node values are positive. If any node has zero or negative value, return 0 immediately. What is the correct approach?

hard

A. Check only first node value, then count all nodes

B. Count nodes first, then check values in a second pass

C. Return length without checking values

D. Traverse nodes, if value <= 0 return 0, else count nodes till end

Solution

1. Step 1: Traverse and check values simultaneously

   Move through each node, check if value is positive.

2. Step 2: Return 0 immediately if value is zero or negative, else keep counting

   If any node fails condition, stop and return 0; otherwise, return total count.

3. Final Answer:

   Traverse nodes, if value <= 0 return 0, else count nodes till end -> Option D

4. Quick Check:

   Check values while counting, stop early if invalid [OK]

Hint: Check values during traversal, stop early if invalid [OK]

Common Mistakes:

• Counting first, then checking values
• Ignoring node values
• Checking only first node value