Circular vs Linear Linked List Key Difference in DSA Java - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if a linked list is circular by comparing the next pointer of the last node.

DSA Java

if (lastNode.next == [1]) {
    System.out.println("List is circular");
} else {
    System.out.println("List is linear");
}

Drag options to blanks, or click blank then click option'

Ahead

Bnull

CfirstNode

DlastNode

Attempts:

3 left

💡 Hint

Common Mistakes

Checking if lastNode.next is null (which is true for linear lists).

Confusing lastNode with head or firstNode.

The last node in a circular linked list points back to the head node, making the list circular.

2fill in blank

medium

Complete the code to traverse a circular linked list starting from head and stopping when it loops back.

DSA Java

Node current = head;
do {
    System.out.print(current.data + " -> ");
    current = current.next;
} while (current != [1]);
System.out.println("null");

Drag options to blanks, or click blank then click option'

Anull

BlastNode

Chead

Dcurrent

Attempts:

3 left

💡 Hint

Common Mistakes

Using null as stopping condition which causes infinite loop.

Using lastNode which may not be accessible during traversal.

In a circular linked list, traversal stops when current reaches head again.

3fill in blank

hard

Fix the error in the method that checks if a linked list is linear by verifying if any node's next is null.

DSA Java

public boolean isLinear(Node head) {
    Node temp = head;
    while (temp != null) {
        if (temp.next == [1]) {
            return true;
        }
        temp = temp.next;
    }
    return false;
}

Drag options to blanks, or click blank then click option'

Ahead

Bnull

Ctemp

Dhead.next

Attempts:

3 left

💡 Hint

Common Mistakes

Checking if next is head which is for circular lists.

Using temp or head.next which are incorrect references.

In a linear linked list, the last node's next pointer is null.

4fill in blank

hard

Fill both blanks to create a method that inserts a new node at the end of a circular linked list.

DSA Java

public void insertEnd(Node newNode) {
    if (head == null) {
        head = newNode;
        newNode.next = [1];
    } else {
        Node temp = head;
        while (temp.next != [2]) {
            temp = temp.next;
        }
        temp.next = newNode;
        newNode.next = head;
    }
}

Drag options to blanks, or click blank then click option'

Ahead

Bnull

CnewNode

Dtemp

Attempts:

3 left

💡 Hint

Common Mistakes

Setting newNode.next to null which breaks circularity.

Using temp or newNode incorrectly in the loop condition.

In a circular linked list, the new node's next points to head, and the last node's next points to the new node.

5fill in blank

hard

Fill all three blanks to create a method that converts a linear linked list to a circular linked list by linking last node to head.

DSA Java

public void makeCircular() {
    if (head == null) return;
    Node temp = head;
    while (temp.next != [1]) {
        temp = temp.next;
    }
    temp.next = [2];
    System.out.println("List converted to circular starting at " + [3].data);
}

Drag options to blanks, or click blank then click option'

Anull

Bhead

Ctemp

Dhead.next

Attempts:

3 left

💡 Hint

Common Mistakes

Using temp or head.next incorrectly in the loop or assignment.

Printing temp.data instead of head.data in the message.

The last node's next is null in linear list; to make circular, link it to head. The message prints head's data.

Practice

1. What is the main difference between a circular linked list and a linear linked list?

easy

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

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

C. A circular linked list has no nodes.

D. A linear linked list always has only one node.

Solution

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

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

2. Step 2: Compare with linear linked list

   In a linear linked list, the last node points to null, so it does not form a loop.

3. Final Answer:

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

4. Quick Check:

   Circular list loops back [OK]

Hint: Circular loops back to start; linear ends with null [OK]

Common Mistakes:

• Confusing which list forms a loop
• Thinking linear list loops back
• Assuming circular list has no nodes

2. Which of the following is the correct way to check if a linked list is circular in Java?

easy

A. if (head != null && head.next != null && head.next == head) { /* circular */ }

B. if (head.next == head) { /* circular */ }

C. if (head.next == null) { /* circular */ }

D. if (head == null) { /* circular */ }

Solution

1. Step 1: Understand circular list check

   In a circular linked list, the last node's next points back to the head node.

2. Step 2: Identify correct condition

   Checking if head.next == head means the list loops back immediately, indicating circularity, but we must also ensure head and head.next are not null to avoid NullPointerException.

3. Final Answer:

   if (head != null && head.next != null && head.next == head) { /* circular */ } -> Option A

4. Quick Check:

   Last node points to head with null checks [OK]

Hint: Check if next points to head for circular list with null safety [OK]

Common Mistakes:

• Checking null instead of loop
• Confusing head and next pointers
• Using incomplete conditions

3. What will be the output of the following Java code snippet?

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

public class Test {
  public static void main(String[] args) {
    Node head = new Node(1);
    Node second = new Node(2);
    Node third = new Node(3);
    head.next = second;
    second.next = third;
    third.next = head; // circular link

    Node temp = head;
    int count = 0;
    while (temp != null && count < 5) {
      System.out.print(temp.data + " ");
      temp = temp.next;
      count++;
    }
  }
}

medium

A. 1 2 3 1 2 3

B. 1 2 3

C. 1 2 3 1 2

D. 1 2 3 3 2

Solution

1. Step 1: Understand the linked list structure

   Nodes 1, 2, 3 are linked linearly, then third node points back to head, making it circular.

2. Step 2: Trace the loop with count limit

   Loop prints node data 5 times: 1, 2, 3, then back to 1, then 2.

3. Final Answer:

   1 2 3 1 2 -> Option C

4. Quick Check:

   5 nodes printed in circular order [OK]

Hint: Count limits circular print to avoid infinite loop [OK]

Common Mistakes:

• Assuming loop stops at null in circular list
• Printing extra nodes beyond count
• Confusing node data order

4. Identify the error in the following code snippet that tries to create a circular linked list:

Node head = new Node(10);
Node second = new Node(20);
head.next = second;
second.next = null; // supposed to make circular

medium

A. head.next should be null to make the list circular.

B. second.next should point to head to make the list circular.

C. Nodes cannot form a circular linked list.

D. second node should not be created separately.

Solution

1. Step 1: Check the pointer assignments

   second.next is set to null, so it does not loop back to head.

2. Step 2: Correct the circular link

   To make the list circular, second.next must point to head node.

3. Final Answer:

   second.next should point to head to make the list circular. -> Option B

4. Quick Check:

   Last node points to head for circular [OK]

Hint: Last node must link to head for circular list [OK]

Common Mistakes:

• Setting last node next to null
• Confusing which node to link back
• Thinking circular list needs no links

5. You want to traverse a circular linked list with unknown length safely in Java. Which approach is best to avoid infinite loops?

hard

A. Traverse until current node's data equals zero.

B. Traverse until current node is null.

C. Traverse a fixed number of nodes, assuming list size is known.

D. Traverse until current node equals the head node again after first visit.

Solution

1. Step 1: Understand circular list traversal

   In circular lists, nodes never point to null, so null check won't stop traversal.

2. Step 2: Use head node as stopping condition

   Stop traversal when current node comes back to head after first visit to avoid infinite loop.

3. Final Answer:

   Traverse until current node equals the head node again after first visit. -> Option D

4. Quick Check:

   Stop when back at head [OK]

Hint: Stop when you reach head again to avoid infinite loop [OK]

Common Mistakes:

• Checking for null in circular list
• Assuming fixed size without knowing it
• Using data value as stop condition