Insert at End of Circular Linked List in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of inserting a node at the end of a circular linked list

What is the printed state of the circular linked list after inserting a node with value 4 at the end?

DSA C#

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

class CircularLinkedList {
    public Node last;

    public CircularLinkedList() { last = null; }

    public void InsertEnd(int data) {
        Node newNode = new Node(data);
        if (last == null) {
            last = newNode;
            last.next = last;
        } else {
            newNode.next = last.next;
            last.next = newNode;
            last = newNode;
        }
    }

    public string PrintList() {
        if (last == null) return "null";
        Node temp = last.next;
        string result = "";
        do {
            result += temp.data + " -> ";
            temp = temp.next;
        } while (temp != last.next);
        result += "null";
        return result;
    }
}

var cll = new CircularLinkedList();
cll.InsertEnd(1);
cll.InsertEnd(2);
cll.InsertEnd(3);
cll.InsertEnd(4);
Console.WriteLine(cll.PrintList());

A null

B 4 -> 1 -> 2 -> 3 -> null

C 1 -> 2 -> 3 -> null

D 1 -> 2 -> 3 -> 4 -> null

💡 Hint

Remember that in a circular linked list, the last node points back to the first node.

The InsertEnd method adds the new node after the last node and updates last to the new node. The list becomes 1 -> 2 -> 3 -> 4 -> null when printed.

❓ Predict Output

intermediate

Output after inserting into an empty circular linked list

What is the printed state of the circular linked list after inserting a node with value 10 into an empty list?

DSA C#

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

class CircularLinkedList {
    public Node last;

    public CircularLinkedList() { last = null; }

    public void InsertEnd(int data) {
        Node newNode = new Node(data);
        if (last == null) {
            last = newNode;
            last.next = last;
        } else {
            newNode.next = last.next;
            last.next = newNode;
            last = newNode;
        }
    }

    public string PrintList() {
        if (last == null) return "null";
        Node temp = last.next;
        string result = "";
        do {
            result += temp.data + " -> ";
            temp = temp.next;
        } while (temp != last.next);
        result += "null";
        return result;
    }
}

var cll = new CircularLinkedList();
cll.InsertEnd(10);
Console.WriteLine(cll.PrintList());

A 10 -> null

B Error

C 10 -> 10 -> null

D null

💡 Hint

Inserting into an empty circular linked list creates a single node pointing to itself.

When the list is empty, the new node points to itself, so printing shows 10 -> null.

🔧 Debug

advanced

Identify the error in InsertEnd method

What error will occur when running this InsertEnd method on a circular linked list?

DSA C#

public void InsertEnd(int data) {
    Node newNode = new Node(data);
    if (last == null) {
        last = newNode;
        last.next = null;
    } else {
        newNode.next = last.next;
        last.next = newNode;
        last = newNode;
    }
}

A NullReferenceException when printing the list

B No error, works correctly

C StackOverflowException due to infinite loop

D Compile-time error due to missing semicolon

💡 Hint

Check what happens when last.next is set to null in a circular linked list.

Setting last.next to null breaks the circular link, so printing the list causes a NullReferenceException when traversing.

❓ Predict Output

advanced

Output after multiple InsertEnd calls

What is the printed state of the circular linked list after these insertions: InsertEnd(5), InsertEnd(7), InsertEnd(9)?

DSA C#

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

class CircularLinkedList {
    public Node last;

    public CircularLinkedList() { last = null; }

    public void InsertEnd(int data) {
        Node newNode = new Node(data);
        if (last == null) {
            last = newNode;
            last.next = last;
        } else {
            newNode.next = last.next;
            last.next = newNode;
            last = newNode;
        }
    }

    public string PrintList() {
        if (last == null) return "null";
        Node temp = last.next;
        string result = "";
        do {
            result += temp.data + " -> ";
            temp = temp.next;
        } while (temp != last.next);
        result += "null";
        return result;
    }
}

var cll = new CircularLinkedList();
cll.InsertEnd(5);
cll.InsertEnd(7);
cll.InsertEnd(9);
Console.WriteLine(cll.PrintList());

A 9 -> 7 -> 5 -> null

B 5 -> 7 -> 9 -> null

C 5 -> 7 -> null

D null

💡 Hint

The last node always points to the first node in a circular linked list.

Each InsertEnd adds a node after last and updates last. The list prints in insertion order: 5 -> 7 -> 9 -> null.

🧠 Conceptual

expert

Why update 'last' pointer when inserting at end in circular linked list?

Why is it necessary to update the 'last' pointer to the new node when inserting at the end of a circular linked list?

A To allow the list to be traversed backwards

B To keep the list sorted in ascending order

C To maintain the reference to the last node so that future insertions at end are efficient

D To prevent the list from becoming singly linked instead of circular

💡 Hint

Think about how the last pointer helps in adding nodes quickly at the end.

Updating 'last' to the new node keeps track of the end of the list, enabling O(1) insertion at the end.

Practice

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

easy

A. The new node points back to the head, maintaining the circular structure.

B. The new node points to null, breaking the circular structure.

C. The new node becomes the head and points to null.

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

Solution

1. Step 1: Understand circular linked list structure

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

2. Step 2: Insert new node at the end

   The new node is linked after the current last node, and its next pointer is set to the head, preserving the circle.

3. Final Answer:

   The new node points back to the head, maintaining the circular structure. -> Option A

4. Quick Check:

   New node next = head keeps list circular [OK]

Hint: New end node must link to head to keep circle [OK]

Common Mistakes:

• Setting new node's next to null breaks the circle
• Inserting new node at head instead of end
• Forgetting to update last node's next pointer

2. Which of the following C# code snippets correctly inserts a new node at the end of a circular linked list?

easy

A. newNode.next = head; lastNode.next = newNode;

B. newNode.next = null; lastNode.next = newNode;

C. newNode.next = lastNode; head.next = newNode;

D. newNode.next = head.next; lastNode.next = head;

Solution

1. Step 1: Set new node's next to head

   To maintain circularity, newNode.next must point to head node.

2. Step 2: Link last node to new node

   Update lastNode.next to newNode to insert at end.

3. Final Answer:

   newNode.next = head; lastNode.next = newNode; -> Option A

4. Quick Check:

   New node points to head, last node points to new node [OK]

Hint: New node next = head; last node next = new node [OK]

Common Mistakes:

• Setting new node next to null breaks circle
• Linking new node to last node instead of head
• Not updating last node's next pointer

3. Given the circular linked list: 10 -> 20 -> 30 -> back to 10, what is the list after inserting 40 at the end?

medium

A. 40 -> 10 -> 20 -> 30 -> back to 40

B. 10 -> 20 -> 30 -> back to 40

C. 10 -> 20 -> 30 -> 40 -> back to 10

D. 10 -> 20 -> 30 -> 40 -> null

Solution

1. Step 1: Identify current circular list

   List is 10 -> 20 -> 30, last node 30 points back to 10.

2. Step 2: Insert 40 at end

   New node 40 is linked after 30, and 40 points back to 10 to keep circle.

3. Final Answer:

   10 -> 20 -> 30 -> 40 -> back to 10 -> Option C

4. Quick Check:

   New node added at end, circle maintained [OK]

Hint: New node added after last, points to head [OK]

Common Mistakes:

• Making new node head instead of last
• Breaking circle by pointing new node to null
• Forgetting to update last node's next pointer

4. What is wrong with this code snippet for inserting at the end of a circular linked list?

newNode.next = null;
lastNode.next = newNode;
newNode.next = head;

medium

A. The code correctly inserts the node without issues.

B. Setting newNode.next to null breaks the circular link temporarily.

C. newNode.next should point to lastNode, not head.

D. lastNode.next should not be updated before newNode.next.

Solution

1. Step 1: Analyze newNode.next assignment

   Setting newNode.next = null breaks the circular link temporarily, which is unnecessary.

2. Step 2: Check order of pointer updates

   Correct insertion requires newNode.next to point to head immediately to maintain circle.

3. Final Answer:

   Setting newNode.next to null breaks the circular link temporarily. -> Option B

4. Quick Check:

   newNode.next must never be null in circular list [OK]

Hint: Never set new node's next to null in circular list [OK]

Common Mistakes:

• Assigning null to newNode.next
• Updating lastNode.next before setting newNode.next
• Confusing pointer directions

5. You have a circular linked list with only one node (value 5) pointing to itself. After inserting a new node with value 10 at the end, what is the correct structure?

hard

A. 5 -> 10 -> null

B. 5 -> back to 10 -> back to 5

C. 10 -> 5 -> back to 10

D. 5 -> 10 -> back to 5

Solution

1. Step 1: Understand single-node circular list

   Initially, node 5 points to itself, forming a circle.

2. Step 2: Insert new node 10 at end

   New node 10 is linked after 5, and 10 points back to 5 to maintain circularity.

3. Final Answer:

   5 -> 10 -> back to 5 -> Option D

4. Quick Check:

   Two nodes linked circularly, last points to head [OK]

Hint: New node next = head even if list has one node [OK]

Common Mistakes:

• Pointing new node to null instead of head
• Not updating original node's next pointer
• Confusing order of nodes after insertion