Create a Circular Singly Linked List in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Circular Singly Linked List?

A Circular Singly Linked List is a linked list where the last node points back to the first node, forming a circle. There is no null at the end.

beginner

How do you identify the last node in a Circular Singly Linked List?

The last node is the one whose next pointer points back to the head (first node) of the list.

intermediate

Why use a Circular Singly Linked List instead of a regular singly linked list?

It allows continuous traversal from any node without stopping, useful for applications like round-robin scheduling or buffering.

intermediate

What is the key difference in node insertion between a regular singly linked list and a circular singly linked list?

In a circular singly linked list, when inserting the first node, its next pointer must point to itself to maintain the circle.

beginner

Show the Python code snippet to create a single node circular singly linked list.

class Node:
    def __init__(self, data):
        self.data = data
        self.next = self

head = Node(10)  # Single node points to itself

In a circular singly linked list, what does the last node's next pointer point to?

A Null

B Itself

C The first node

D The second node

The last node's next pointer points back to the first node to form a circle.

What is the main advantage of a circular singly linked list?

A No need to check for null to end traversal

B Uses less memory

C Faster search

D Simpler insertion at head

Traversal can continue indefinitely without checking for null because the list loops back to the start.

When creating the first node in a circular singly linked list, what should its next pointer be set to?

A Itself

B Null

C Head

D None

The first node points to itself to maintain the circular structure.

Which of these is NOT a property of a circular singly linked list?

A Last node points to first node

B Has a null pointer at the end

C Each node has one next pointer

D Traversal can loop infinitely

Circular singly linked lists do not have a null pointer at the end.

How do you detect the end of traversal in a circular singly linked list?

A When next pointer points to itself

B When next pointer is null

C When data is None

D When you reach the head node again

Traversal ends when you come back to the head node, completing the circle.

Explain how to create a circular singly linked list with one node and why the next pointer points to itself.

Think about how to keep the list circular even with one node.

Describe the difference between a regular singly linked list and a circular singly linked list in terms of traversal and end condition.

Focus on what happens when you reach the last node.

Practice

1. What is a key characteristic of a circular singly linked list compared to a regular singly linked list?

easy

A. It uses an array internally instead of nodes.

B. The last node points back to the first node, forming a circle.

C. Each node has two pointers, one to the next and one to the previous node.

D. The list has a fixed size and cannot grow.

Solution

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

   In a circular singly linked list, the last node's next pointer points back to the first node, creating a loop.

2. Step 2: Compare with other list types

   Unlike doubly linked lists, nodes have only one pointer. Unlike arrays, nodes are linked dynamically. The circular nature is unique to this structure where the last node points back to the first node, forming a circle.

3. Final Answer:

   The last node points back to the first node, forming a circle. -> Option B

4. Quick Check:

   Circular list loops back = C [OK]

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

Common Mistakes:

• Confusing circular with doubly linked list
• Thinking nodes have two pointers
• Assuming fixed size like arrays

2. Which of the following is the correct way to initialize an empty circular singly linked list in Python?

easy

A. head = None

B. head = Node(0); head.next = None

C. head = Node(0); head.next = head

D. head = []

Solution

1. Step 1: Understand empty list initialization

   An empty circular singly linked list has no nodes, so head should be None.

2. Step 2: Analyze other options

   head = Node(0); head.next = None sets next to None, which is not circular. head = Node(0); head.next = head creates a node pointing to itself, which is a single-node circular list, not empty. head = [] uses a list, not a linked list.

3. Final Answer:

   head = None -> Option A

4. Quick Check:

   Empty list head = None [OK]

Hint: Empty list means head points to nothing (None) [OK]

Common Mistakes:

• Setting head.next to None in empty list
• Creating a node when list is empty
• Using Python list [] instead of nodes

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(1)
second = Node(2)
third = Node(3)

head.next = second
second.next = third
third.next = head

current = head
result = []
for _ in range(4):
    result.append(current.data)
    current = current.next
print(result)

medium

A. [1, 2, 3, 3]

B. [1, 2, 3, None]

C. [1, 2, 3, 1]

D. RuntimeError due to infinite loop

Solution

1. Step 1: Understand the circular link setup

   Nodes 1, 2, 3 are linked in a circle: 1->2->3->1.

2. Step 2: Trace the loop for 4 iterations

   Starting at head (1), the loop collects data: 1, then moves to 2, then 3, then back to 1.

3. Final Answer:

   [1, 2, 3, 1] -> Option C

4. Quick Check:

   Loop of 3 nodes repeated once = [1,2,3,1] [OK]

Hint: Follow next pointers carefully for given loop count [OK]

Common Mistakes:

• Assuming last node points to None
• Stopping after 3 elements instead of 4
• Confusing data values with node references

4. Identify the error in the following code that tries to create a circular singly linked list with one node:

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

head = Node(10)
head.next = None  # Intended to point to head for circular list

medium

A. Data value 10 is invalid for circular lists

B. Node class is missing a previous pointer

C. head should be initialized as None

D. head.next should point to head, not None

Solution

1. Step 1: Check the pointer assignment for circularity

   For a circular singly linked list with one node, the node's next must point to itself (head), not None.

2. Step 2: Analyze the given code

   The code sets head.next = None, breaking the circular link.

3. Final Answer:

   head.next should point to head, not None -> Option D

4. Quick Check:

   Single node circular list loops to itself [OK]

Hint: Single node circular list next points to itself [OK]

Common Mistakes:

• Setting next to None instead of head
• Confusing singly with doubly linked list
• Thinking data value affects circularity

5. You want to insert a new node with value 4 at the end of an existing circular singly linked list with nodes 1 -> 2 -> 3 -> back to 1. Which of the following code snippets correctly performs this insertion?

hard

A. new_node = Node(4)
current = head
while current.next != head:
current = current.next
current.next = new_node
new_node.next = head

B. new_node = Node(4)
new_node.next = head
head.next = new_node

C. new_node = Node(4)
head.next = new_node
new_node.next = None

D. new_node = Node(4)
current = head
while current.next != None:
current = current.next
current.next = new_node
new_node.next = head

Solution

1. Step 1: Locate the last node in the circular list

   Traverse nodes until current.next points back to head, identifying the last node.

2. Step 2: Insert new node after last node and link it back to head

   Set last node's next to new_node, then new_node's next to head to maintain circularity.

3. Final Answer:

   Traverse until current.next != head, then set current.next = new_node and new_node.next = head -> Option A

4. Quick Check:

   Traverse till last node, link new node, maintain circle [OK]

Hint: Find last node by checking next != head, then insert [OK]

Common Mistakes:

• Setting new_node.next to None breaking circularity
• Not traversing to last node correctly
• Assuming head.next is always last node