Create a Circular Singly Linked List in DSA Typescript - Execution Trace

Concept Flow - Create a Circular Singly Linked List

Create new node

↓

Is list empty?

No→Traverse to last node

↓

Set last.next = new node

↓

Set head = new node

↓

Set new node.next = head

↓

Done

Start by creating a new node. If the list is empty, set head to new node and point new node to itself. Otherwise, find the last node and link it to the new node, then link new node back to head.

Execution Sample

DSA Typescript

class Node {
  value: number;
  next: Node | null = null;
  constructor(value: number) { this.value = value; }
}

class CircularLinkedList {
  head: Node | null = null;
  add(value: number) {
    const newNode = new Node(value);
    if (!this.head) {
      this.head = newNode;
      newNode.next = newNode;
    } else {
      let current = this.head;
      while (current.next !== this.head) {
        current = current.next!;
      }
      current.next = newNode;
      newNode.next = this.head;
    }
  }
}

This code adds nodes to a circular singly linked list, linking the last node back to the head.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Create new node with value 10	None	newNode created (10)	null
2	List empty? Yes, set head = newNode and newNode.next = newNode	10	head -> 10, 10.next -> 10	10 -> 10 (points to itself)
3	Create new node with value 20	10	newNode created (20)	10 -> 10
4	Traverse to last node (10)	10	current -> 10	10 -> 10
5	Set last.next = newNode (20)	10, 20	10.next -> 20	10 -> 20
6	Set newNode.next = head (10)	10, 20	20.next -> 10	10 -> 20 -> 10 (circular)
7	Create new node with value 30	10, 20	newNode created (30)	10 -> 20 -> 10
8	Traverse to last node (10 -> 20)	10, 20	current moves: 10 -> 20	10 -> 20 -> 10
9	Set last.next = newNode (30)	10, 20, 30	20.next -> 30	10 -> 20 -> 30
10	Set newNode.next = head (10)	10, 20, 30	30.next -> 10	10 -> 20 -> 30 -> 10 (circular)
11	End of add operations	10, 20, 30	No pointer changes	10 -> 20 -> 30 -> 10 (circular)

💡 No more nodes to add, list is circular with last node pointing to head

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	After Step 9	After Step 10	Final
head	null	null	Node(10)	Node(10)	Node(10)	Node(10)	Node(10)	Node(10)	Node(10)	Node(10)	Node(10)	Node(10)
newNode	null	Node(10)	Node(10)	Node(20)	Node(20)	Node(20)	Node(20)	Node(30)	Node(30)	Node(30)	Node(30)	null
current	null	null	null	null	Node(10)	Node(10)	Node(10)	null	Node(20)	Node(20)	Node(20)	null

Key Moments - 3 Insights

Why do we set newNode.next = newNode when the list is empty?

Because in a circular singly linked list with one node, the node must point to itself to maintain the circular property. See step 2 in execution_table where newNode.next points to itself.

Why do we traverse until current.next === head instead of current === null?

Because the list is circular, there is no null at the end. The last node points back to head, so we stop traversal when current.next is head. See steps 4 and 8 where traversal stops at last node.

How does the list maintain circularity after adding new nodes?

After adding a new node, we set the last node's next to the new node and the new node's next back to head, preserving the circle. See steps 5, 6, 9, and 10.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what does newNode.next point to?

A It points to head which is null

B It points to null

C It points to itself (newNode)

D It points to the next new node

💡 Hint

Check the Pointer Changes and Visual State columns at step 2 in execution_table

At which step does the list first become circular?

A Step 1

B Step 2

C Step 5

D Step 10

💡 Hint

Look for when newNode.next points to itself or head in execution_table

If we add a node with value 40 after step 10, what will be the new last node's next pointer?

A It will point to the new node (40)

B It will point to null

C It will point to head (10)

D It will point to the previous last node (30)

💡 Hint

Refer to steps 6 and 10 where last.next is set to newNode and newNode.next is set to head

Concept Snapshot

Create a Circular Singly Linked List:
- Create new node
- If list empty: head = new node, newNode.next = newNode (points to itself)
- Else: traverse to last node (where next points to head)
- Set last.next = newNode
- Set newNode.next = head
- List is circular: last node points back to head

Full Transcript

To create a circular singly linked list, start by creating a new node. If the list is empty, set the head to this new node and make the new node point to itself to form a circle. If the list is not empty, find the last node by traversing until the node whose next points to head. Then, link this last node's next to the new node, and link the new node's next back to head. This keeps the list circular. Each addition updates pointers carefully to maintain the circle. The traversal stops when current.next equals head because the list has no null end. This method ensures the list always loops back to the start.

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 uses an array internally to store nodes.

D. The list always has a fixed size.

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 list types

   Regular singly linked lists end with a null pointer, doubly linked lists have two pointers per node, and arrays are not linked lists.

3. Final Answer:

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

4. Quick Check:

   Circular list last node points to head = A [OK]

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

Common Mistakes:

• Thinking circular means doubly linked
• Assuming last node points to null
• Confusing arrays with linked lists

2. Which of the following TypeScript code snippets correctly initializes an empty circular singly linked list?

easy

A. let head: Node = null; head.next = head;

B. let head: Node | null = null;

C. let head: Node | null = new Node(null); head.next = null;

D. let head: Node = new Node(null); head.next = head;

Solution

1. Step 1: Understand initialization of circular singly linked list

   For an empty circular singly linked list with one node, the node's next pointer should point to itself.

2. Step 2: Analyze each option

   let head: Node = new Node(null); head.next = head; creates a node and sets its next to itself, correctly forming a circular list with one node. let head: Node | null = null; sets head to null (empty list, but not circular). let head: Node | null = new Node(null); head.next = null; sets next to null (not circular). let head: Node = null; head.next = head; tries to assign next on null (error).

3. Final Answer:

   let head: Node = new Node(null); head.next = head; -> Option D

4. Quick Check:

   One node points to itself = A [OK]

Hint: One node circular list points next to itself [OK]

Common Mistakes:

• Setting next to null instead of self
• Assigning next on null variable
• Not creating a node before linking

3. Given the following TypeScript code for a circular singly linked list, what will be the output after inserting nodes with values 1, 2, and 3 in order?

class Node {
  value: number;
  next: Node | null;
  constructor(value: number) {
    this.value = value;
    this.next = null;
  }
}

class CircularList {
  head: Node | null = null;

  insert(value: number) {
    const newNode = new Node(value);
    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;
    }
  }

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

const list = new CircularList();
list.insert(1);
list.insert(2);
list.insert(3);
console.log(list.print());

medium

A. 1 -> 2 -> 3 -> 1 -> null

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

C. 1 -> 2 -> 3 -> null

D. 1 -> 2 -> 3

Solution

1. Step 1: Trace insertions and list structure

   Insert 1: head points to node 1, node 1 next points to itself.
   Insert 2: find last node (1), set last node's next to new node 2, new node 2 next points to head (1).
   Insert 3: find last node (2), set last node's next to new node 3, new node 3 next points to head (1).

2. Step 2: Trace print method output

   Print starts at head (1), prints 1 -> 2 -> 3 -> then stops when it reaches head again, appends 'null'.

3. Final Answer:

   1 -> 2 -> 3 -> null -> Option C

4. Quick Check:

   Print stops at head, adds null = B [OK]

Hint: Print stops when node loops back to head [OK]

Common Mistakes:

• Printing extra head node again
• Missing 'null' at end
• Infinite loop without stopping condition

4. Identify the bug in this TypeScript method for inserting a node into a circular singly linked list:

insert(value: number) {
  const newNode = new Node(value);
  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. The method does not create a new node correctly.

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

C. The while loop condition should be temp.next === null.

D. The head should be set to null after insertion.

Solution

1. Step 1: Analyze insertion when list is empty

   When the list is empty, newNode.next is set to null, breaking circular property. It should point to head to form a circle.

2. Step 2: Check loop condition and other logic

   The while loop correctly finds the last node by checking temp.next !== head. Other parts are correct.

3. Final Answer:

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

4. Quick Check:

   Empty list node next must point to itself = C [OK]

Hint: Empty list node next must point to itself, not null [OK]

Common Mistakes:

• Setting next to null breaks circular link
• Wrong loop condition causes infinite loop
• Resetting head incorrectly

5. You want to implement a method delete(value: number) to remove a node with a given value from a circular singly linked list. Which approach correctly handles deleting the head node while maintaining the circular structure?

hard

A. Find the last node, set its next to head.next, then update head to head.next.

B. Set head to null directly without updating any other node.

C. Set head.next to null and keep head unchanged.

D. Remove the node by setting head to head.next without updating last node's next.

Solution

1. Step 1: Understand deleting head in circular list

   To delete the head, we must update the last node's next pointer to the new head (head.next) to maintain the circle.

2. Step 2: Analyze each option

   Find the last node, set its next to head.next, then update head to head.next. correctly finds last node, updates its next to head.next, then moves head to head.next.
   Set head to null directly without updating any other node. breaks the list by setting head to null without fixing last node.
   Set head.next to null and keep head unchanged. breaks the circle by setting head.next to null.
   Remove the node by setting head to head.next without updating last node's next. moves head but leaves last node pointing to old head, breaking the circle.

3. Final Answer:

   Find the last node, set its next to head.next, then update head to head.next. -> Option A

4. Quick Check:

   Last node next updated to new head = D [OK]

Hint: Update last node's next when deleting head [OK]

Common Mistakes:

• Not updating last node's next pointer
• Setting head to null without cleanup
• Breaking circular link by null assignments