Why Trees Exist and What Linked Lists and Arrays Cannot Do in DSA Typescript - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a tree node with a value and empty children array.

DSA Typescript

class TreeNode {
  value: number;
  children: TreeNode[];
  constructor(val: number) {
    this.value = val;
    this.children = [1];
  }
}

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

A[]

B{}

Cnull

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using {} which creates an object instead of an array.

Setting children to null or 0 which are not arrays.

The children property should be an empty array to hold child nodes.

2fill in blank

medium

Complete the code to add a child node to a tree node's children array.

DSA Typescript

function addChild(parent: TreeNode, child: TreeNode) {
  parent.children.[1](child);
}

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

Ashift

Bpush

Cpop

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() which removes the last element instead of adding.

Using shift() or unshift() which affect the start of the array.

Use push() to add a new child node at the end of the children array.

3fill in blank

hard

Fix the error in the code to check if a node has children.

DSA Typescript

function hasChildren(node: TreeNode): boolean {
  return node.children.length [1] 0;
}

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

A==

B!=

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using == 0 which checks for no children.

Using != 0 which is less clear than > 0.

We check if the children array length is greater than 0 to know if there are children.

4fill in blank

hard

Fill both blanks to create a function that counts total nodes in a tree recursively.

DSA Typescript

function countNodes(node: TreeNode): number {
  let count = 1;
  for (const child of node.children) {
    count += countNodes([1]);
  }
  return count;
}

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

Anode

Bcount

Cchildren

Dchild

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'node' which causes infinite recursion on the same node.

Using 'children' which is an array, not a node.

We recursively count nodes by calling countNodes on each child node.

5fill in blank

hard

Fill all three blanks to create a function that finds a node by value in a tree.

DSA Typescript

function findNode(node: TreeNode, val: number): TreeNode | null {
  if (node.value === [1]) return node;
  for (const child of node.[2]) {
    const found = findNode(child, [3]);
    if (found) return found;
  }
  return null;
}

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

Aval

Bchildren

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'value' instead of 'val' in comparisons.

Looping over 'value' instead of 'children'.

Passing wrong variable in recursive call.

We compare node.value to val, loop over children, and pass val recursively.

Practice

1. Why do trees exist as a data structure when we already have arrays and linked lists?

easy

A. Because trees are easier to implement than arrays and linked lists.

B. Because trees use less memory than arrays and linked lists.

C. Because trees always have faster access times than arrays and linked lists.

D. Because trees can represent hierarchical relationships which arrays and linked lists cannot.

Solution

1. Step 1: Understand the limitation of arrays and linked lists

   Arrays and linked lists store data in a linear sequence, which makes it hard to represent hierarchical or branching relationships.

2. Step 2: Identify the unique feature of trees

   Trees allow each element to have multiple children, representing hierarchical data like file systems or organization charts.

3. Final Answer:

   Trees represent hierarchical relationships which arrays and linked lists cannot -> Option D

4. Quick Check:

   Hierarchical data needs trees = C [OK]

Hint: Trees model hierarchies; arrays and lists are linear [OK]

Common Mistakes:

• Thinking trees always use less memory
• Believing trees are simpler to implement
• Assuming trees always have faster access

2. Which of the following is the correctly way to declare a simple tree node in TypeScript?

easy

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

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

C. class TreeNode { value: number; prev: TreeNode; constructor(value: number) { this.value = value; this.prev = null; } }

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

Solution

1. Step 1: Identify tree node properties

   A tree node stores a value and multiple children nodes, so it needs a children array.

2. Step 2: Match with the correct class declaration

   class TreeNode { value: number; children: TreeNode[]; constructor(value: number) { this.value = value; this.children = []; } } declares a value and children array, suitable for a tree node. Declarations using next or prev are for linked lists. class TreeNode { value: number; array: number[]; constructor(value: number) { this.value = value; this.array = []; } } uses an array of numbers, not nodes.

3. Final Answer:

   correctly declares a tree node with children array -> Option A

4. Quick Check:

   Tree nodes have children array = A [OK]

Hint: Tree nodes have children arrays, not next or prev pointers [OK]

Common Mistakes:

• Confusing tree nodes with linked list nodes
• Using next or prev instead of children array
• Using array of numbers instead of nodes

3. What will be the output of the following TypeScript code?

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);
root.children.push(child2);
console.log(root.children.length);

medium

A. 0

B. 1

C. 2

D. 3

Solution

1. Step 1: Understand the tree construction

   A root node with value 1 is created. Two child nodes with values 2 and 3 are created and added to root's children array.

2. Step 2: Count the children of root

   Two children are pushed into root.children, so root.children.length is 2.

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Two children added = 2 [OK]

Hint: Count how many children are pushed into the children array [OK]

Common Mistakes:

• Assuming children length is 0 by default
• Confusing children with linked list next pointers
• Counting nodes instead of children array length

4. Find the error in this TypeScript code that tries to create a tree node and add children:

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

const root = new TreeNode(10);
root.children.push(new TreeNode(20));

medium

A. The constructor should not have parameters.

B. children should be initialized as an empty array, not null.

C. The push method does not exist on arrays.

D. TreeNode class should not have a children property.

Solution

1. Step 1: Check children initialization

   children is set to null, so calling push on null causes an error.

2. Step 2: Correct children initialization

   children should be initialized as an empty array [] to allow push method calls.

3. Final Answer:

   children should be initialized as an empty array, not null -> Option B

4. Quick Check:

   push on null causes error = A [OK]

Hint: Initialize children as [] before pushing nodes [OK]

Common Mistakes:

• Setting children to null instead of []
• Trying to push on null causing runtime error
• Ignoring constructor parameters

5. You want to represent a company's organizational chart where each employee can have multiple subordinates. Which data structure is best and why?

Choose the best answer.

hard

A. Use a tree because it naturally models hierarchical relationships with multiple children per node.

B. Use a linked list because it allows easy insertion and deletion of employees.

C. Use a stack because it follows last-in-first-out which suits organizational charts.

D. Use an array because it stores data sequentially and is easy to access.

Solution

1. Step 1: Understand the problem context

   An organizational chart is hierarchical: managers have multiple subordinates, forming branches.

2. Step 2: Match data structure to hierarchy

   Trees allow nodes to have multiple children, perfectly modeling this hierarchy. Arrays and linked lists are linear, stacks are LIFO structures unsuitable here.

3. Final Answer:

   Use a tree because it naturally models hierarchical relationships -> Option A

4. Quick Check:

   Hierarchy needs trees = D [OK]

Hint: Hierarchies need trees, not linear or LIFO structures [OK]

Common Mistakes:

• Choosing arrays or linked lists for hierarchical data
• Confusing stack behavior with organizational needs
• Ignoring multiple children per node requirement