Delete a Node from 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. There is no null at the end.

beginner

Why is deleting a node from a circular linked list different from a normal linked list?

Because the last node points back to the head, we must update pointers carefully to keep the circle intact after deletion.

intermediate

What steps are needed to delete the head node in a circular linked list?

1. Find the last node (which points to head). 2. Change last node's next to head's next. 3. Update head to head's next. 4. Remove old head.

intermediate

How do you delete a node with a given value in a circular linked list?

Traverse the list to find the node with the value. Keep track of previous node. Update previous node's next to skip the node to delete. Handle special case if node is head.

beginner

What happens if you try to delete a node not present in the circular linked list?

The list remains unchanged because the node to delete was not found after a full traversal.

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

A The first node

B Null

C Itself

D The previous node

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

What must you update when deleting the head node in a circular linked list?

A The last node's next pointer and the head pointer

B Only the head pointer

C Only the last node's next pointer

D No pointers need updating

You must update the last node's next pointer to the new head and update the head pointer itself.

If the node to delete is not found in the circular linked list, what happens?

A The list is deleted

B The list remains unchanged

C The head is deleted

D The last node is deleted

If the node is not found, no changes are made to the list.

Which pointer do you need to keep track of when deleting a node other than the head?

A Next node's next pointer

B Head pointer

C Tail pointer

D Previous node's next pointer

You keep track of the previous node to update its next pointer to skip the deleted node.

What is the first step to delete a node with a specific value in a circular linked list?

A Delete the head node

B Create a new node

C Traverse the list to find the node

D Set the last node's next to null

You must first find the node with the given value by traversing the list.

Explain how to delete the head node from a circular linked list step-by-step.

Think about how the circle is maintained after removing the first node.

Describe the process to delete a node with a given value from a circular linked list.

Remember to keep the circle intact and update pointers carefully.

Practice

1. What happens when you delete the head node from a circular linked list with multiple nodes?

easy

A. The head moves to the next node and the list remains circular

B. The list becomes empty

C. The last node is deleted instead

D. The list breaks and becomes linear

Solution

1. Step 1: Understand the head deletion in circular linked list

   When the head node is deleted, the next node becomes the new head.

2. Step 2: Maintain circular link

   The last node's next pointer is updated to point to the new head, keeping the list circular.

3. Final Answer:

   The head moves to the next node and the list remains circular -> Option A

4. Quick Check:

   Head deletion updates head and keeps circular [OK]

Hint: Deleting head shifts head to next, list stays circular [OK]

Common Mistakes:

• Assuming list becomes empty after head deletion
• Not updating last node's next pointer
• Breaking circular link accidentally

2. Which of the following is the correct syntax to delete a node with value `key` from a circular linked list in Python?

easy

A. def deleteNode(head, key):
if head.data == key:
# delete head logic

B. def deleteNode(head, key):
while head.data != key:
head = head.next

C. def deleteNode(head, key):
if head is None:
return None

D. def deleteNode(head, key):
head = None
return head

Solution

1. Step 1: Check for empty list

   Before deleting, check if the list is empty (head is None) to avoid errors.

2. Step 2: Return None if list is empty

   If head is None, return None as there is nothing to delete.

3. Final Answer:

   def deleteNode(head, key): if head is None: return None -> Option C

4. Quick Check:

   Empty list check is essential before deletion [OK]

Hint: Always check if list is empty before deleting [OK]

Common Mistakes:

• Not checking if head is None before deletion
• Incorrectly looping without exit condition
• Setting head to None without condition

3. What will be the output list after deleting node with value 3 from this circular linked list: 1 -> 3 -> 5 -> 7 -> (back to 1)?

medium

A. 1 -> 5 -> 7 -> 3 -> (back to 1)

B. 1 -> 5 -> 7 -> (back to 1)

C. 1 -> 3 -> 5 -> 7 -> (back to 1)

D. 3 -> 5 -> 7 -> (back to 3)

Solution

1. Step 1: Locate node with value 3

   Traverse the list to find node with value 3 between 1 and 5.

2. Step 2: Remove node 3 and link 1 to 5

   Update node 1's next pointer to node 5, skipping node 3.

3. Final Answer:

   1 -> 5 -> 7 -> (back to 1) -> Option B

4. Quick Check:

   Deleted 3, linked 1 to 5 [OK]

Hint: Skip the node with key, link previous to next [OK]

Common Mistakes:

• Not updating previous node's next pointer
• Deleting wrong node or not deleting at all
• Breaking circular link after deletion

4. Given this code snippet to delete a node from a circular linked list, what is the bug?

def deleteNode(head, key):
    if head is None:
        return None
    curr = head
    while curr.next != head:
        if curr.next.data == key:
            curr.next = curr.next.next
            return head
        curr = curr.next
    return head

medium

A. Does not handle deletion if head node contains the key

B. Infinite loop due to missing break

C. Deletes all nodes instead of one

D. Returns None instead of head

Solution

1. Step 1: Analyze deletion logic

   The code checks nodes after head but never checks if head itself has the key.

2. Step 2: Identify missing head deletion handling

   If head contains the key, this code will not delete it, causing incorrect behavior.

3. Final Answer:

   Does not handle deletion if head node contains the key -> Option A

4. Quick Check:

   Head node deletion must be handled separately [OK]

Hint: Always check if head node matches key before loop [OK]

Common Mistakes:

• Ignoring head node in deletion logic
• Assuming loop covers all nodes including head
• Not updating head pointer after deletion

5. You have a circular linked list with only one node containing value 10. What should be the list after deleting node with value 10?

hard

A. List with node 10 pointing to None

B. List with node 10 pointing to itself

C. List unchanged

D. Empty list (None)

Solution

1. Step 1: Identify single node list condition

   When the list has only one node, head.next points to head itself.

2. Step 2: Delete the only node

   Deleting this node means the list becomes empty, so head should be None.

3. Final Answer:

   Empty list (None) -> Option D

4. Quick Check:

   Single node deletion empties the list [OK]

Hint: Deleting sole node results in empty list (None) [OK]

Common Mistakes:

• Leaving node pointing to itself after deletion
• Not setting head to None
• Assuming list still has one node