Insert at Beginning of Circular Linked List in DSA Java - 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 without a null end.

intermediate

Why do we need a special method to insert at the beginning of a circular linked list?

Because the last node points to the first node, inserting at the beginning requires updating the last node's next pointer to the new node to maintain the circle.

intermediate

What are the main steps to insert a new node at the beginning of a circular linked list?

1. Create a new node.
2. If list is empty, point new node to itself.
3. Otherwise, find last node.
4. Point new node's next to current head.
5. Update last node's next to new node.
6. Update head to new node.

beginner

What happens if you insert at the beginning of an empty circular linked list?

The new node points to itself, becoming the only node in the list, and the head points to this new node.

beginner

In Java, how do you update the head of a circular linked list after insertion at the beginning?

After adjusting the last node's next pointer to the new node, assign the head reference to the new node to mark it as the first node.

What does the last node in a circular linked list point to?

A The first node

B Null

C Itself

D The second node

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

When inserting at the beginning of a circular linked list, what must be updated besides the head?

A The second node's next pointer

B The last node's next pointer

C The tail pointer only

D No other pointer

The last node's next pointer must point to the new head to maintain the circular structure.

If the circular linked list is empty, what should the new node's next pointer be set to?

A Head

B Null

C Itself

D Last node

In an empty circular linked list, the new node points to itself to form a single-node circle.

Which of these is NOT a step when inserting at the beginning of a circular linked list?

A Set the new node's next to null

B Find the last node

C Update the last node's next pointer

D Update the head to the new node

The new node's next pointer should point to the old head, not null.

What is the time complexity to insert at the beginning of a circular linked list if you do not maintain a tail pointer?

A O(1)

B O(n^2)

C O(log n)

D O(n)

Without a tail pointer, you must traverse the list to find the last node, which takes O(n) time.

Explain how to insert a new node at the beginning of a circular linked list.

Think about how the circle is maintained when adding a new first node.

Describe what changes in the pointers when inserting at the beginning of a circular linked list compared to a singly linked list.

Focus on the circular nature and how it affects pointer updates.

Practice

1. What happens when you insert a new node at the beginning of a circular linked list?

easy

A. The new node becomes the head and points to the old head, maintaining the circular link.

B. The new node is added at the end of the list, breaking the circular link.

C. The new node replaces the last node in the list.

D. The list becomes linear and loses its circular property.

Solution

1. Step 1: Understand the circular linked list structure

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

2. Step 2: Insert new node at beginning

   The new node is linked to the current head, and the last node's next pointer is updated to this new node, making it the new head.

3. Final Answer:

   The new node becomes the head and points to the old head, maintaining the circular link. -> Option A

4. Quick Check:

   New head points to old head, circle intact = C [OK]

Hint: New head points to old head, last node points to new head [OK]

Common Mistakes:

• Forgetting to update last node's next pointer
• Breaking the circular link by not linking last node
• Replacing last node instead of adding new head

2. Which of the following Java code snippets correctly inserts a new node at the beginning of a circular linked list?

easy

A. last.next = head; head = newNode; newNode.next = last;

B. head.next = newNode; newNode.next = last; last = newNode;

C. newNode.next = null; head = newNode; last.next = head;

D. newNode.next = head; last.next = newNode; head = newNode;

Solution

1. Step 1: Link new node to current head

   Set newNode.next = head to point new node to the current head.

2. Step 2: Update last node's next to new node and move head

   Set last.next = newNode to maintain circular link, then update head = newNode.

3. Final Answer:

   newNode.next = head; last.next = newNode; head = newNode; -> Option D

4. Quick Check:

   Correct pointer updates = A [OK]

Hint: Link new node to head, update last.next, then move head [OK]

Common Mistakes:

• Setting newNode.next to null breaks circular link
• Not updating last.next to new node
• Incorrect order of pointer updates

3. What will be the output of the following code snippet after inserting a node with value 10 at the beginning of the circular linked list?

class Node {
  int data;
  Node next;
  Node(int d) { data = d; next = null; }
}

Node head = new Node(20);
Node second = new Node(30);
head.next = second;
second.next = head; // circular link

Node newNode = new Node(10);
// Insert at beginning
newNode.next = head;
second.next = newNode;
head = newNode;

// Print list starting from head
Node temp = head;
do {
  System.out.print(temp.data + " -> ");
  temp = temp.next;
} while(temp != head);
System.out.print("null");

medium

A. 10 -> 20 -> 30 -> null

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

C. 10 -> 30 -> 20 -> null

D. 30 -> 20 -> 10 -> null

Solution

1. Step 1: Understand initial list and insertion

   Initial list: 20 -> 30 -> back to 20. New node 10 inserted at beginning, so head moves to 10.

2. Step 2: Trace the print loop

   Starting at head (10), print 10, then 20, then 30, then loop ends at head again.

3. Final Answer:

   10 -> 20 -> 30 -> null -> Option A

4. Quick Check:

   New head 10 followed by old nodes = D [OK]

Hint: Start printing from new head, follow next pointers until back to head [OK]

Common Mistakes:

• Printing nodes in wrong order
• Not updating last node's next pointer
• Confusing circular link with null termination

4. Identify the error in the following code that attempts to insert a new node at the beginning of a circular linked list:

Node newNode = new Node(5);
newNode.next = head;
head = newNode;
// Missing update of last node's next pointer

medium

A. The head should not be updated before updating last node's next pointer.

B. The new node's next pointer should be null instead of head.

C. The last node's next pointer is not updated to point to the new head, breaking the circular link.

D. The new node should be inserted at the end, not the beginning.

Solution

1. Step 1: Check pointer updates after insertion

   New node points to old head, and head is updated, but last node still points to old head.

2. Step 2: Identify circular link break

   Since last node's next is not updated to new head, the circular link breaks, causing incorrect list traversal.

3. Final Answer:

   The last node's next pointer is not updated to point to the new head, breaking the circular link. -> Option C

4. Quick Check:

   Last node must point to new head to keep circle [OK]

Hint: Always update last.next to new head after insertion [OK]

Common Mistakes:

• Forgetting to update last node's next pointer
• Assuming head update is enough
• Not maintaining circular property

5. Given a circular linked list with nodes 5 -> 15 -> 25 -> back to 5, write the sequence of node values after inserting a new node with value 1 at the beginning. Assume the insertion updates all pointers correctly. What is the resulting list?

hard

A. 5 -> 15 -> 25 -> 1 -> null

B. 1 -> 5 -> 15 -> 25 -> null

C. 1 -> 15 -> 25 -> 5 -> null

D. 25 -> 5 -> 15 -> 1 -> null

Solution

1. Step 1: Insert new node at beginning

   New node with value 1 points to old head (5), and last node (25) points to new node.

2. Step 2: Update head pointer

   Head is updated to new node (1), so traversal starts from 1.

3. Final Answer:

   1 -> 5 -> 15 -> 25 -> null -> Option B

4. Quick Check:

   New head 1 followed by old nodes in order [OK]

Hint: New node becomes head, last node points to new head [OK]

Common Mistakes:

• Not updating last node's next pointer
• Incorrect order of nodes after insertion
• Confusing circular list with linear list