Traversal of Circular Linked List in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Circular Linked List Traversal

What is the printed output of the following code that traverses a circular linked list starting from the head node?

DSA Python

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

# Create nodes
head = Node(10)
second = Node(20)
third = Node(30)

# Link nodes in circular fashion
head.next = second
second.next = third
third.next = head

# Traverse circular linked list
current = head
output = []
while True:
    output.append(str(current.data))
    current = current.next
    if current == head:
        break
print(' -> '.join(output) + ' -> null')

A 10 -> 20 -> null

B 10 -> 20 -> 30 -> 10 -> null

C RuntimeError: infinite loop

D 10 -> 20 -> 30 -> null

💡 Hint

Remember that traversal stops when we reach the head node again.

The traversal starts at head (10), moves to 20, then 30, and stops before repeating head again. So output is '10 -> 20 -> 30 -> null'.

🧠 Conceptual

intermediate

Number of Nodes in Circular Linked List

Given a circular linked list where the head node points back to itself after the last node, how many nodes will be counted by traversing until you reach the head again?

A The number of nodes in the list

B One less than the number of nodes

C Infinite nodes because of the loop

D Zero nodes because head is repeated

💡 Hint

Traversal stops when you reach the head again, so count all unique nodes once.

Traversal counts each node once until it reaches the head again, so the count equals the total number of nodes.

🔧 Debug

advanced

Identify the Bug in Circular Linked List Traversal

What error will occur when running this traversal code on a circular linked list?

DSA Python

current = head
while current.next != head:
    print(current.data, end=' -> ')
    current = current.next
print(current.data, end=' -> null')

A No error, prints all nodes correctly

B Skips the last node in the list

C Infinite loop occurs

D AttributeError because current is None

💡 Hint

Check if the last node's data is printed or skipped.

The while loop runs while current.next != head, printing data for all nodes except the last inside the loop, then prints the last node's data (' -> null') after the loop. All nodes are printed correctly with no error.

📝 Syntax

advanced

Identify Syntax Error in Circular Linked List Traversal

Which option contains a syntax error when trying to traverse a circular linked list?

DSA Python

current = head
while True
    print(current.data)
    current = current.next
    if current == head:
        break

A No syntax error

B Missing colon after while True

C Incorrect comparison operator in if statement

D Missing parentheses in print statement

💡 Hint

Check the syntax of the while loop line.

The while loop line is missing a colon at the end, causing a syntax error.

🚀 Application

expert

Result of Modifying Circular Linked List During Traversal

Consider this code that removes the node with data 20 during traversal of a circular linked list. What is the printed output after modification?

DSA Python

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

head = Node(10)
second = Node(20)
third = Node(30)
head.next = second
second.next = third
third.next = head

current = head
prev = None
output = []
while True:
    if current.data == 20:
        prev.next = current.next
        current = current.next
        continue
    output.append(str(current.data))
    prev = current
    current = current.next
    if current == head:
        break
print(' -> '.join(output) + ' -> null')

A RuntimeError due to NoneType attribute access

B 10 -> 20 -> 30 -> null

C 10 -> 30 -> null

D Infinite loop without stopping

💡 Hint

Trace the values of prev and current step by step, especially during deletion and the continue statement.

When current reaches 20, prev is 10, so prev.next = 30 removes 20 safely. The continue skips appending 20. Then appends 10 (earlier) and 30, stopping before head again. Output: 10 -> 30 -> null.

Practice

1. What is the main difference between a circular linked list and a regular singly linked list?

easy

A. In a circular linked list, the last node points back to the first node.

B. In a circular linked list, nodes have two pointers instead of one.

C. In a circular linked list, nodes are stored in an array.

D. In a circular linked list, the first node points to null.

Solution

1. Step 1: Understand the structure of a circular linked list

   A circular linked list connects the last node back to the first node, forming a circle.

2. Step 2: Compare with a regular singly linked list

   A regular singly linked list ends with a node pointing to null, not back to the first node.

3. Final Answer:

   In a circular linked list, the last node points back to the first node. -> Option A

4. Quick Check:

   Circular linked list last node points to first node [OK]

Hint: Remember: circular means last node loops to first [OK]

Common Mistakes:

• Thinking circular lists have two pointers per node
• Confusing circular linked list with doubly linked list
• Assuming last node points to null

2. Which of the following is the correct way to start traversing a circular linked list in Python?

easy

A. current = head
while current is not None:
print(current.data)
current = current.next

B. current = head
while True:
print(current.data)
current = current.next
if current == head:
break

C. current = head
for i in range(len(list)):
print(current.data)
current = current.next

D. current = head
while current.next is not None:
print(current.data)
current = current.next

Solution

1. Step 1: Understand traversal stopping condition for circular list

   Traversal must stop when current node comes back to head to avoid infinite loop.

2. Step 2: Check code options for correct loop and break condition

   current = head while True: print(current.data) current = current.next if current == head: break uses while True with break when current equals head, correctly handling circular traversal.

3. Final Answer:

   Use while True loop and break when current equals head. -> Option B

4. Quick Check:

   Stop traversal when current == head [OK]

Hint: Use break when current node equals head to stop [OK]

Common Mistakes:

• Using None as stopping condition in circular list
• Forgetting to break loop causing infinite traversal
• Using length which is not defined for linked list

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

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

head = Node(10)
second = Node(20)
third = Node(30)

head.next = second
second.next = third
third.next = head  # circular link

current = head
count = 0
while True:
    print(current.data, end=' ')
    current = current.next
    count += 1
    if count == 4:
        break

medium

A. 10 20 30 30

B. 10 20 30

C. 10 20 30 10

D. 10 20 20 10

Solution

1. Step 1: Trace the nodes and their links

   Nodes are 10 -> 20 -> 30 -> back to 10 forming a circle.

2. Step 2: Follow the loop for 4 iterations

   Prints: 10 (count=1), 20 (2), 30 (3), 10 (4) then breaks.

3. Final Answer:

   10 20 30 10 -> Option C

4. Quick Check:

   4 prints following circular links [OK]

Hint: Count iterations to avoid infinite loop in circular list [OK]

Common Mistakes:

• Stopping after 3 prints assuming no repetition
• Printing wrong node data due to pointer confusion
• Ignoring circular link causing infinite loop

4. The following code is intended to traverse a circular linked list and print all node data once. What is the error?

current = head
while current != head:
    print(current.data)
    current = current.next

medium

A. The code will raise an AttributeError.

B. The loop will run infinitely because no break condition.

C. The code will print only the head node data once.

D. The loop never starts because current == head initially.

Solution

1. Step 1: Analyze the loop condition at start

   current is initialized to head, so current != head is False initially.

2. Step 2: Understand loop execution

   Since condition is False, loop body never executes, so no data is printed.

3. Final Answer:

   The loop never starts because current == head initially. -> Option D

4. Quick Check:

   Loop condition false at start stops loop [OK]

Hint: Check initial loop condition carefully in circular traversal [OK]

Common Mistakes:

• Assuming loop runs at least once
• Using != head as loop condition without special handling
• Not using do-while style logic for circular lists

5. You have a circular linked list with nodes containing values [5, 15, 25, 35]. Write a Python function to traverse and collect all node values in a list without infinite looping. What is the correct output of your function?

hard

A. [5, 15, 25, 35]

B. [5, 15, 25, 35, 5]

C. [15, 25, 35, 5]

D. [5, 15, 25]

Solution

1. Step 1: Traverse circular list starting at head

   Start at head node with value 5 and move to next nodes collecting values.

2. Step 2: Stop traversal when current node returns to head

   Collect values 5, 15, 25, 35 and stop before repeating 5 again.

3. Final Answer:

   [5, 15, 25, 35] -> Option A

4. Quick Check:

   Collect all unique nodes once [OK]

Hint: Stop when current node equals head to avoid repeats [OK]

Common Mistakes:

• Including head node twice causing extra element
• Stopping too early missing last node
• Infinite loop by not checking for head again