Node Structure and Pointer Design in DSA Typescript - Execution Trace

Concept Flow - Node Structure and Pointer Design

Create Node with data

↓

Set node.next to null

↓

Link node to list via pointers

↓

Update pointers (head, tail) as needed

↓

Traverse list using pointers

↓

Access or modify node data

↓

Repeat or end

This flow shows how a node is created with data and a pointer, linked into a list, and how pointers help traverse and modify the list.

Execution Sample

DSA Typescript

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

Defines a Node class with a data field and a next pointer initialized to null.

Execution Table

Step	Operation	Node Created	Pointer Changes	Visual State
1	Create node with data=10	Node(data=10, next=null)	No pointer changes yet	┌────────┐ │ data:10│ │ next:∅ │ └────────┘
2	Create node with data=20	Node(data=20, next=null)	No pointer changes yet	┌────────┐ ┌────────┐ │ data:10│ │ data:20│ │ next:∅ │ │ next:∅ │ └────────┘ └────────┘
3	Link first node's next to second node	No new node created	node10.next → node20	┌────────┐ ┌────────┐ │ data:10│ ──→│ data:20│ │ next:──┼ │ next:∅ │ └────────┘ └────────┘
4	Create node with data=30	Node(data=30, next=null)	No pointer changes yet	┌────────┐ ┌────────┐ ┌────────┐ │ data:10│ ──→│ data:20│ │ data:30│ │ next:──┼ │ next:∅ │ │ next:∅ │ └────────┘ └────────┘ └────────┘
5	Link second node's next to third node	No new node created	node20.next → node30	┌────────┐ ┌────────┐ ┌────────┐ │ data:10│ ──→│ data:20│ ──→│ data:30│ │ next:──┼ │ next:──┼ │ next:∅ │ └────────┘ └────────┘ └────────┘
6	Traverse list from head	No new node created	current pointer moves node10 → node20 → node30 → null	Traversal order: 10 → 20 → 30 → ∅
7	End traversal	No new node created	current pointer is null, stop	Traversal complete

💡 Traversal ends when current pointer reaches null (no next node).

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
node10	null	Node(10,null)	Node(10,null)	Node(10,node20)	Node(10,node20)	Node(10,node20)	Node(10,node20)	Node(10,node20)
node20	null	null	Node(20,null)	Node(20,null)	Node(20,null)	Node(20,node30)	Node(20,node30)	Node(20,node30)
node30	null	null	null	null	Node(30,null)	Node(30,null)	Node(30,null)	Node(30,null)
current	null	null	null	null	null	null	node10 → node20 → node30 → null	null

Key Moments - 3 Insights

Why is the 'next' pointer initialized to null when creating a node?

Because at creation, the node is not linked to any other node yet. See execution_table step 1 and 2 where next is ∅ (null). This prevents accidental references.

How do pointers change when linking nodes?

Pointers update to point from one node to the next. For example, in step 3, node10.next points to node20, shown by the arrow ──→ in the visual state.

Why does traversal stop when current pointer is null?

Because null means no next node exists. In step 7, current becomes null, so traversal ends safely without errors.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what does node10.next point to?

A null

B node30

C node20

D node10

💡 Hint

Check the Pointer Changes and Visual State columns at step 3.

At which step does the list first have three nodes linked together?

A Step 4

B Step 5

C Step 6

D Step 3

💡 Hint

Look for the Visual State showing three nodes connected with arrows.

If we forget to set node.next to null in the constructor, what might happen?

A The node will point to random memory causing errors

B The node will automatically link to the next node

C The node will be isolated with no next pointer

D The list will be empty

💡 Hint

Think about why next is explicitly set to null in the code sample.

Concept Snapshot

Node Structure and Pointer Design:
- Node holds data and a pointer (next) to another node or null.
- next pointer initialized to null to avoid accidental links.
- Linking nodes updates next pointers to form chains.
- Traversal uses pointers to move from node to node.
- Traversal ends when next pointer is null.

Full Transcript

This concept covers how nodes are structured with data and pointers, specifically the next pointer in a singly linked list. Each node is created with data and its next pointer set to null initially. Nodes are linked by updating the next pointer of one node to point to another node. This linking forms a chain of nodes. Traversal of the list uses these pointers to move from one node to the next until the pointer is null, indicating the end of the list. The execution steps show node creation, linking, and traversal with pointer updates and visual diagrams. Key points include why next is set to null at creation and how pointers change during linking and traversal.

Practice

1. What does the next pointer in a singly linked list node represent?

easy

A. It points to the previous node in the list

B. It stores the data value of the node

C. It holds the size of the list

D. It points to the next node in the list

Solution

1. Step 1: Understand the role of next in singly linked list

   The next pointer is used to link the current node to the following node in the list.

2. Step 2: Differentiate from other pointers

   Unlike prev which points backward, next always points forward to the next node.

3. Final Answer:

   It points to the next node in the list -> Option D

4. Quick Check:

   next pointer = next node [OK]

Hint: Remember: next means forward link in singly linked list [OK]

Common Mistakes:

• Confusing next with previous pointer
• Thinking next stores data instead of address
• Assuming next holds list size

2. Which of the following is the correct TypeScript syntax to define a node class with a data property and a next pointer?

easy

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

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

C. class Node { data: number; next: Node; constructor() { this.data = 0; this.next = null; } }

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

Solution

1. Step 1: Check property types and initialization

   The next pointer should allow null to represent the end of the list, so its type must be Node | null.

2. Step 2: Verify constructor assigns values correctly

   The constructor takes data and sets next to null initially.

3. Final Answer:

   class Node { data: number; next: Node | null; constructor(data: number) { this.data = data; this.next = null; } } -> Option B

4. Quick Check:

   Allow null for next pointer = class Node { data: number; next: Node | null; constructor(data: number) { this.data = data; this.next = null; } } [OK]

Hint: Next pointer must allow null to mark list end [OK]

Common Mistakes:

• Not allowing next to be null
• Forgetting to initialize next in constructor
• Using wrong types for next pointer

3. Given the following TypeScript code, what will be the output when printing the linked list data values?

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

const head = new Node(10);
head.next = new Node(20);
head.next.next = new Node(30);

let current = head;
const result: number[] = [];
while (current !== null) {
  result.push(current.data);
  current = current.next;
}
console.log(result);

medium

A. [10, 20, 30]

B. [10, 20]

C. [20, 30]

D. []

Solution

1. Step 1: Trace node creation and linking

   Nodes are created with data 10, 20, and 30. The next pointers link 10 -> 20 -> 30 -> null.

2. Step 2: Follow the while loop to collect data

   The loop starts at head (10), adds 10, moves to 20, adds 20, moves to 30, adds 30, then stops at null.

3. Final Answer:

   [10, 20, 30] -> Option A

4. Quick Check:

   Traverse all nodes = [10, 20, 30] [OK]

Hint: Traverse until next is null to collect all nodes [OK]

Common Mistakes:

• Stopping traversal too early
• Not linking nodes correctly
• Confusing data values with pointers

4. Identify the error in the following TypeScript code for a singly linked list node and fix it:

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

medium

A. Data property should be optional

B. The constructor should not assign null to next

C. The type of next should be 'Node | null' to allow null assignment

D. Class should not have a constructor

Solution

1. Step 1: Analyze the type of next pointer

   The next property is typed as Node but assigned null, which is invalid.

2. Step 2: Correct the type to allow null

   Change the type to Node | null so it can hold either a node or null.

3. Final Answer:

   The type of next should be 'Node | null' to allow null assignment -> Option C

4. Quick Check:

   Allow null in next type = The type of next should be 'Node | null' to allow null assignment [OK]

Hint: Next pointer must allow null for list end [OK]

Common Mistakes:

• Not allowing null in next type
• Removing constructor unnecessarily
• Making data optional without reason

5. You want to design a doubly linked list node in TypeScript that stores a number and has pointers to both previous and next nodes. Which of the following class definitions correctly implements this with proper pointer types and initialization?

hard

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

B. class Node { data: number; prev: Node; next: Node; constructor(data: number) { this.data = data; this.prev = null; this.next = null; } }

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

D. class Node { data: number; prev: null; next: null; constructor(data: number) { this.data = data; this.prev = null; this.next = null; } }

Solution

1. Step 1: Check pointer types for doubly linked list

   Both prev and next pointers must allow null to represent ends of the list.

2. Step 2: Verify constructor initializes pointers to null

   Constructor correctly sets prev and next to null initially.

3. Final Answer:

   class Node { data: number; prev: Node | null; next: Node | null; constructor(data: number) { this.data = data; this.prev = null; this.next = null; } } -> Option A

4. Quick Check:

   Doubly linked list pointers allow null = class Node { data: number; prev: Node | null; next: Node | null; constructor(data: number) { this.data = data; this.prev = null; this.next = null; } } [OK]

Hint: Both prev and next pointers must allow null [OK]

Common Mistakes:

• Not allowing null for prev or next
• Setting pointer types incorrectly
• Initializing pointers to wrong values