Get Length of Linked List in DSA Python - 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 (pointer) to the next node. It is like a treasure hunt where each clue leads to the next.

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 indexing to find length in a linked list like arrays?

Linked lists don't store elements in continuous memory, so we can't jump to a position directly. We must visit each node to count.

intermediate

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

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

beginner

Show the Python code snippet to get the length of a linked list.

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

What does each node in a linked list contain?

A Only data

B Only a pointer to the next node

C Data and a pointer to the next node

D Data and index number

Each node stores data and a pointer to the next node to form the chain.

How do you know you reached the end of a linked list?

A When the next pointer is null

B When the data is zero

C When the index is last

D When the node points to itself

The last node's next pointer is null, indicating the end.

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

A O(1)

B O(n)

C O(log n)

D O(n^2)

You must visit each node once, so time is proportional to n.

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

A Because nodes are not stored in continuous memory

B Because the 5th node is hidden

C Because linked lists do not have indexes

D Because linked lists are sorted

Nodes are scattered in memory, so we must follow links one by one.

What variable is commonly used to keep track of the count while finding length?

A size

B length

C index

D count

A variable like 'count' is used to increment as we visit each node.

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 Python 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 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