Get Length of Linked List in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the length of the linked list after these insertions?

Consider a singly linked list where we insert nodes with values 10, 20, and 30 at the end. What will be the length of the linked list after these insertions?

DSA Python

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

class LinkedList:
    def __init__(self):
        self.head = None

    def append(self, data):
        new_node = Node(data)
        if not self.head:
            self.head = new_node
            return
        last = self.head
        while last.next:
            last = last.next
        last.next = new_node

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

ll = LinkedList()
ll.append(10)
ll.append(20)
ll.append(30)
length = ll.get_length()
print(length)

A 3

B 2

C 0

D 1

💡 Hint

Count each node starting from the head until you reach the end (None).

The linked list has three nodes inserted: 10, 20, and 30. The get_length method counts all nodes, so the length is 3.

❓ Predict Output

intermediate

What is the length of the linked list after deleting one node?

Given a linked list with nodes 5 -> 15 -> 25 -> None, if we delete the head node, what will be the length of the linked list?

DSA Python

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

class LinkedList:
    def __init__(self):
        self.head = None

    def append(self, data):
        new_node = Node(data)
        if not self.head:
            self.head = new_node
            return
        last = self.head
        while last.next:
            last = last.next
        last.next = new_node

    def delete_head(self):
        if self.head:
            self.head = self.head.next

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

ll = LinkedList()
ll.append(5)
ll.append(15)
ll.append(25)
ll.delete_head()
length = ll.get_length()
print(length)

A 3

B 2

C 0

D 1

💡 Hint

Deleting the head removes the first node, so count remaining nodes.

Initially, the list has 3 nodes. After deleting the head (5), the list becomes 15 -> 25 -> None, which has length 2.

🔧 Debug

advanced

What error occurs when calling get_length on an empty linked list?

Consider the following code where no nodes are added. What happens when get_length is called?

DSA Python

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

class LinkedList:
    def __init__(self):
        self.head = None

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

ll = LinkedList()
print(ll.get_length())

A AttributeError

B NoneType error

C 0

D 1

💡 Hint

An empty list has no nodes, so count should be zero.

The head is None, so the while loop does not run and count remains 0. No error occurs.

🧠 Conceptual

advanced

What is the time complexity of the get_length method for a linked list with n nodes?

The get_length method counts nodes by traversing the linked list from head to end. What is its time complexity?

A O(n^2) quadratic time

B O(1) constant time

C O(log n) logarithmic time

D O(n) linear time

💡 Hint

The method visits each node once.

The method visits each of the n nodes once, so the time grows linearly with n.

🚀 Application

expert

After multiple insertions and deletions, what is the final length of the linked list?

Starting with an empty linked list, perform these operations in order: 1. Append 1 2. Append 2 3. Append 3 4. Delete head 5. Append 4 6. Delete head 7. Append 5 What is the length of the linked list now?

DSA Python

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

class LinkedList:
    def __init__(self):
        self.head = None

    def append(self, data):
        new_node = Node(data)
        if not self.head:
            self.head = new_node
            return
        last = self.head
        while last.next:
            last = last.next
        last.next = new_node

    def delete_head(self):
        if self.head:
            self.head = self.head.next

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

ll = LinkedList()
ll.append(1)
ll.append(2)
ll.append(3)
ll.delete_head()
ll.append(4)
ll.delete_head()
ll.append(5)
length = ll.get_length()
print(length)

A 3

B 4

C 5

D 2

💡 Hint

Track the list after each operation carefully.

After operations, the list is 3 -> 4 -> 5 -> None, length is 3.

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