Create a Circular Singly Linked List in DSA Javascript - 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. This means you can traverse the list starting from any node and eventually come back to it.

beginner

How do you identify the end of a Circular Singly Linked List during traversal?

You identify the end when you reach the starting node again. Since there is no null pointer, the list loops back to the start, so you stop when the current node is the same as the starting node.

intermediate

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

Because it allows continuous traversal without needing to reset to the head. It is useful for applications like round-robin scheduling or buffering where you cycle through elements repeatedly.

beginner

In JavaScript, how do you create a node for a Circular Singly Linked List?

You create a node as an object with two properties: 'data' to store the value and 'next' to point to the next node. For example: const node = { data: value, next: null };

beginner

What is the key step to make a singly linked list circular after adding nodes?

The key step is to set the 'next' pointer of the last node to point back to the head node, closing the loop.

What does the 'next' pointer of the last node in a Circular Singly Linked List point to?

A The first node (head)

B Null

C Itself

D The second node

In a Circular Singly Linked List, the last node's 'next' pointer points back to the first node, forming a circle.

How do you know when to stop traversing a Circular Singly Linked List?

A When current node is null

B After visiting all nodes once, when current node equals the start node

C After a fixed number of steps

D When current node's data is zero

Since the list is circular, traversal stops when you reach the starting node again after visiting all nodes.

Which of these is NOT a benefit of Circular Singly Linked Lists?

A Simpler to implement than arrays

B No need to reset to head for repeated cycles

C Easy continuous traversal

D Useful for round-robin scheduling

Circular lists are not necessarily simpler than arrays; they have specific use cases but can be more complex to implement.

In JavaScript, what is the minimal structure of a node in a Circular Singly Linked List?

A A function returning the next node

B An array with two elements

C A string representing the value

D An object with 'data' and 'next' properties

A node is an object with 'data' to hold the value and 'next' to point to the next node.

What happens if you forget to link the last node back to the head in a Circular Singly Linked List?

A The list will have a cycle anyway

B The list becomes doubly linked

C The list becomes a regular singly linked list

D The list will be empty

Without linking the last node back to the head, the list is just a regular singly linked list ending with null.

Explain how to create a Circular Singly Linked List in JavaScript from scratch.

Think about how to connect the last node back to the first.

Describe the difference between a regular singly linked list and a circular singly linked list.

Focus on the last node's pointer and traversal stopping condition.

Practice

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

easy

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

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

C. It stores data in a sorted order automatically.

D. It uses an array internally to store nodes.

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 linked lists

   Unlike doubly linked lists, circular singly linked lists have only one pointer per node and form a circle, not a linear chain.

3. Final Answer:

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

4. Quick Check:

   Last node's next = first node [OK]

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

Common Mistakes:

• Confusing circular singly linked list with doubly linked list
• Thinking nodes have two pointers
• Assuming data is sorted automatically

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

easy

A. const head = { data: null, next: head };

B. const head = null; // no nodes yet

C. const head = { data: null, next: null };

D. const head = []; // empty array

Solution

1. Step 1: Understand empty list initialization

   An empty circular singly linked list has no nodes, so the head pointer is null.

2. Step 2: Check other options

   const head = { data: null, next: null }; creates a node with null data and next pointer, which is not empty. const head = { data: null, next: head }; references head before assignment, causing error. const head = []; // empty array uses an array, not a linked list.

3. Final Answer:

   const head = null; // no nodes yet -> Option B

4. Quick Check:

   Empty list head = null [OK]

Hint: Empty list means head is null, no nodes created [OK]

Common Mistakes:

• Creating a node with null data instead of null head
• Referencing head before declaration
• Using arrays instead of linked list nodes

3. What will be the output of the following code snippet that creates a circular singly linked list with nodes 1, 2, 3 and prints the list once?

class Node {
  constructor(data) {
    this.data = data;
    this.next = null;
  }
}

class CircularSinglyLinkedList {
  constructor() {
    this.head = null;
  }

  append(data) {
    const newNode = new Node(data);
    if (!this.head) {
      this.head = newNode;
      newNode.next = this.head;
    } else {
      let temp = this.head;
      while (temp.next !== this.head) {
        temp = temp.next;
      }
      temp.next = newNode;
      newNode.next = this.head;
    }
  }

  printList() {
    if (!this.head) return '';
    let result = '';
    let temp = this.head;
    do {
      result += temp.data + ' -> ';
      temp = temp.next;
    } while (temp !== this.head);
    result += 'null';
    return result;
  }
}

const list = new CircularSinglyLinkedList();
list.append(1);
list.append(2);
list.append(3);
console.log(list.printList());

medium

A. null

B. 1 -> 2 -> 3 -> 1 -> 2 -> 3 -> null

C. 1 -> 2 -> 3

D. 1 -> 2 -> 3 -> null

Solution

1. Step 1: Understand append method behavior

   Each new node is added at the end, and its next points to head, maintaining circularity.

2. Step 2: Trace printList method

   printList starts at head and loops until it reaches head again, appending data with ' -> '. Finally adds 'null'. So output is '1 -> 2 -> 3 -> null'.

3. Final Answer:

   1 -> 2 -> 3 -> null -> Option D

4. Quick Check:

   Print once around circle = 1 -> 2 -> 3 -> null [OK]

Hint: Print loops until back to head once, then stops [OK]

Common Mistakes:

• Printing infinite loop by not stopping at head
• Missing 'null' at end
• Printing nodes multiple times

4. Identify the bug in this code snippet that tries to append a node to a circular singly linked list:

append(data) {
  const newNode = new Node(data);
  if (!this.head) {
    this.head = newNode;
    newNode.next = null;
  } else {
    let temp = this.head;
    while (temp.next !== this.head) {
      temp = temp.next;
    }
    temp.next = newNode;
    newNode.next = this.head;
  }
}

medium

A. newNode.next should point to this.head when list is empty, not null.

B. The while loop condition should be temp.next === this.head.

C. The head should be set to newNode every time a node is appended.

D. The newNode should not be created inside the append method.

Solution

1. Step 1: Check initialization for empty list

   When list is empty, newNode.next must point to head itself to maintain circularity, not null.

2. Step 2: Verify loop and pointer updates

   The while loop and pointer updates are correct; only the initial next pointer is wrong.

3. Final Answer:

   newNode.next should point to this.head when list is empty, not null. -> Option A

4. Quick Check:

   Empty list node next = head, not null [OK]

Hint: Empty list node next must loop to itself [OK]

Common Mistakes:

• Setting newNode.next to null breaks circular link
• Wrong while loop condition
• Resetting head incorrectly

5. You want to insert a new node with value 4 after the node with value 3 in a circular singly linked list. Given the list: 3 -> 1 -> 2 -> (back to 3), which code snippet correctly performs this insertion?

hard

A. let temp = head;
while (temp.next !== head) {
temp = temp.next;
}
const newNode = new Node(4);
temp.next = newNode;
newNode.next = head;

B. const newNode = new Node(4);
newNode.next = head;
head.next = newNode;

C. let temp = head;
while (temp.data !== 3) {
temp = temp.next;
}
const newNode = new Node(4);
newNode.next = temp.next;
temp.next = newNode;

D. head = new Node(4);
head.next = oldHead;

Solution

1. Step 1: Locate node with value 3

   Traverse from head until temp.data equals 3 to find the correct insertion point.

2. Step 2: Insert new node after node 3

   Create newNode with data 4, set newNode.next to temp.next, then set temp.next to newNode to insert correctly.

3. Final Answer:

   Traverse from head until finding node with value 3, then insert new node after it. -> Option C

4. Quick Check:

   Insert after node 3 by adjusting pointers [OK]

Hint: Find node 3, insert new node by pointer swap [OK]

Common Mistakes:

• Inserting at wrong position
• Not updating pointers correctly
• Replacing head incorrectly