Traversal of Circular Linked List in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a circular linked list?

A circular linked list is a linked list where the last node points back to the first node, forming a circle with no end.

beginner

How do you know when to stop traversing a circular linked list?

You stop when you reach the starting node again after moving through the list, to avoid infinite loops.

beginner

Why can't you use 'null' to stop traversal in a circular linked list?

Because in a circular linked list, there is no 'null' at the end; the last node points back to the first node.

intermediate

Show the basic Python code snippet to traverse a circular linked list starting from the head node.

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

beginner

What is the main difference between traversing a singly linked list and a circular linked list?

In a singly linked list, traversal stops when the next node is null. In a circular linked list, traversal stops when you return to the starting node.

What condition is used to stop traversal in a circular linked list?

A When current node is null

B After visiting 10 nodes

C When current node equals the head node again

D When current node's next is null

Traversal stops when the current node is the head node again, indicating a full circle.

In a circular linked list, what does the last node point to?

A Null

B The head node

C A random node

D The previous node

The last node points back to the head node to form a circle.

Why is it important to check if current == head during traversal?

A To avoid infinite loops

B To find the middle node

C To count the nodes

D To delete the head node

Checking current == head prevents infinite loops by stopping traversal after one full cycle.

What happens if you forget to check current == head in traversal?

A Traversal throws an error

B Traversal stops early

C Traversal skips nodes

D Traversal never ends (infinite loop)

Without the check, traversal loops endlessly because the list is circular.

Which data structure is best described as a circular linked list?

A A list where last node points to the first node

B A list where last node points to null

C A list with no nodes

D A list with nodes pointing randomly

A circular linked list has its last node pointing back to the first node.

Explain how to traverse a circular linked list and how to avoid infinite loops.

Think about what makes circular linked lists different from normal linked lists.

Describe the key difference between singly linked list traversal and circular linked list traversal.

Focus on how the end of the list is identified in each case.

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