Tree vs Array vs Linked List When Hierarchy Matters in DSA Typescript - Compare & Choose

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Traversing a Simple Tree Structure

What is the output of the following TypeScript code that traverses a simple tree and prints node values in preorder?

DSA Typescript

class TreeNode {
  value: number;
  children: TreeNode[];
  constructor(value: number) {
    this.value = value;
    this.children = [];
  }
}

const root = new TreeNode(1);
const child1 = new TreeNode(2);
const child2 = new TreeNode(3);
root.children.push(child1, child2);
child1.children.push(new TreeNode(4));

function preorder(node: TreeNode | null, result: number[] = []): number[] {
  if (!node) return result;
  result.push(node.value);
  for (const child of node.children) {
    preorder(child, result);
  }
  return result;
}

console.log(preorder(root).join(' -> ') + ' -> null');

A 1 -> 3 -> 2 -> 4 -> null

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

C 4 -> 2 -> 3 -> 1 -> null

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

💡 Hint

Preorder traversal visits the current node before its children, and children are visited in the order they appear.

The preorder traversal visits root first (1), then its first child (2), then 2's child (4), then the second child of root (3).

🧠 Conceptual

intermediate

Choosing Data Structure for Hierarchical Data

Which data structure is best suited to represent a company's organizational hierarchy where each employee can have multiple subordinates?

A Tree, because it naturally represents hierarchical relationships with parent and multiple children

B Array, because it stores elements in order and allows fast access by index

C Linked List, because it allows easy insertion and deletion of employees in a sequence

D Stack, because it manages elements in last-in-first-out order

💡 Hint

Think about how to represent multiple levels and branches of employees.

A tree structure is ideal for hierarchical data because each node can have multiple children representing subordinates.

🔧 Debug

advanced

Identify the Error in Linked List Node Insertion

What error will occur when running this TypeScript code that tries to insert a node at the head of a singly linked list?

DSA Typescript

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

let head: ListNode | null = null;

function insertAtHead(value: number) {
  const newNode = new ListNode(value);
  newNode.next = head;
  head = newNode;
}

insertAtHead(10);
insertAtHead(20);
console.log(head.next.value);

A 20

B undefined

C 10

D TypeError: Cannot read property 'value' of null

💡 Hint

Trace the insertions and what head.next points to after two insertions.

After inserting 10 then 20 at head, head points to 20, head.next points to 10, so head.next.value is 10.

❓ Predict Output

advanced

Output of Flattening a Tree into an Array

What is the output of this TypeScript code that flattens a tree into an array using depth-first search?

DSA Typescript

class TreeNode {
  value: number;
  children: TreeNode[];
  constructor(value: number) {
    this.value = value;
    this.children = [];
  }
}

const root = new TreeNode(1);
const c1 = new TreeNode(2);
const c2 = new TreeNode(3);
const c3 = new TreeNode(4);
root.children.push(c1, c2);
c1.children.push(c3);

function flattenTree(node: TreeNode | null, arr: number[] = []): number[] {
  if (!node) return arr;
  arr.push(node.value);
  for (const child of node.children) {
    flattenTree(child, arr);
  }
  return arr;
}

console.log(flattenTree(root));

A [1, 2, 4, 3]

B [1, 3, 2, 4]

C [4, 2, 3, 1]

D [2, 4, 1, 3]

💡 Hint

Depth-first search visits node before children recursively.

The flattenTree function visits root (1), then child 2, then 2's child 4, then child 3.

🧠 Conceptual

expert

Why Not Use Array for Deep Hierarchical Data?

Why is using a simple array a poor choice to represent deeply nested hierarchical data compared to a tree or linked list?

A Arrays cannot store multiple data types, so they are unsuitable for hierarchy

B Arrays automatically sort elements, which breaks hierarchical order

C Arrays are slower than trees for all operations, so they are inefficient

D Arrays do not naturally represent parent-child relationships and require complex indexing to simulate hierarchy

💡 Hint

Think about how hierarchy needs explicit parent-child links.

Arrays store elements linearly and lack direct links to represent nested parent-child relationships, making hierarchy representation complex.

Practice

1. Which data structure is best suited to represent hierarchical relationships like a family tree or company organization?

easy

A. Tree

B. Array

C. Linked List

D. Stack

Solution

1. Step 1: Understand hierarchy representation

   Hierarchies have parent-child relationships, which are naturally modeled by trees.

2. Step 2: Compare data structures

   Arrays and linked lists are linear and do not represent branching well, unlike trees.

3. Final Answer:

   Tree -> Option A

4. Quick Check:

   Hierarchy = Tree [OK]

Hint: Hierarchy means branching, so choose Tree [OK]

Common Mistakes:

• Choosing Array because it's simple
• Choosing Linked List thinking it can represent hierarchy
• Confusing Stack with hierarchical structure

2. Which of the following is the correct TypeScript syntax to declare a simple linked list node with a value and a next pointer?

easy

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

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

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

D. class Node { value: number; next: any; constructor(value: number) { this.value = value; this.next = undefined; } }

Solution

1. Step 1: Check next pointer type

   Next pointer should allow either another Node or null to mark list end.

2. Step 2: Verify constructor initialization

   Constructor sets value and initializes next to null correctly.

3. Final Answer:

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

4. Quick Check:

   Next pointer = Node or null [OK]

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

Common Mistakes:

• Not allowing null for next pointer
• Initializing next as undefined instead of null
• Using any type which loses type safety

3. Given the following TypeScript code, what will be the printed output?

class TreeNode {
  value: number;
  children: TreeNode[];
  constructor(value: number) {
    this.value = value;
    this.children = [];
  }
}

const root = new TreeNode(1);
const child1 = new TreeNode(2);
const child2 = new TreeNode(3);
root.children.push(child1, child2);
console.log(root.children.map(c => c.value));

medium

A. [1, 2, 3]

B. Error: children is not iterable

C. [[2], [3]]

D. [2, 3]

Solution

1. Step 1: Understand tree structure

   Root node has value 1 and two children with values 2 and 3.

2. Step 2: Analyze console.log output

   Mapping children array extracts values [2, 3], root's value 1 is not included.

3. Final Answer:

   [2, 3] -> Option D

4. Quick Check:

   Children values = [2, 3] [OK]

Hint: Mapping children prints their values, not root's [OK]

Common Mistakes:

• Including root value in output
• Expecting nested arrays instead of flat array
• Assuming children is not iterable

4. Identify the bug in this TypeScript code that tries to create a linked list and print its values:

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

const head = new Node(1);
head.next = new Node(2);
head.next.next = new Node(3);

let current = head;
while (current !== null) {
  console.log(current.value);
  current = current.next.next;
}

medium

A. Using console.log inside loop is invalid

B. Incorrect constructor syntax

C. Accessing next.next without checking if next is null

D. Node class missing a previous pointer

Solution

1. Step 1: Trace the while loop iteration

   Loop moves current by current.next.next, skipping nodes and risking null access.

2. Step 2: Identify null pointer risk

   When current.next is null, accessing current.next.next causes runtime error.

3. Final Answer:

   Accessing next.next without checking if next is null -> Option C

4. Quick Check:

   Null check missing before next.next access [OK]

Hint: Always check next before accessing next.next [OK]

Common Mistakes:

• Ignoring null checks causing runtime errors
• Thinking constructor syntax is wrong
• Believing console.log causes errors

5. You need to represent a company's organizational chart where each employee can have multiple direct reports. Which data structure should you use and why? Choose the best option and explain your reasoning.

hard

A. Tree, because it naturally models hierarchical relationships with multiple children

B. Linked List, because it allows dynamic insertion and deletion of employees

C. Stack, because it supports last-in-first-out operations suitable for reporting

D. Array, because it stores elements in order and is easy to access by index

Solution

1. Step 1: Analyze the problem requirements

   Organizational chart has hierarchy with employees having multiple direct reports (children).

2. Step 2: Match data structure to hierarchy

   Tree structure supports nodes with multiple children, representing managers and their reports well.

3. Step 3: Evaluate other options

   Array and linked list are linear and do not represent branching; stack is for LIFO, not hierarchy.

4. Final Answer:

   Tree, because it naturally models hierarchical relationships with multiple children -> Option A

5. Quick Check:

   Hierarchy with multiple children = Tree [OK]

Hint: Hierarchy with multiple children means Tree is best [OK]

Common Mistakes:

• Choosing Array for hierarchy
• Thinking Linked List handles multiple children well
• Confusing Stack with hierarchical data