BST Insert Operation in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new node with the given value.

DSA Typescript

class Node {
  value: number;
  left: Node | null = null;
  right: Node | null = null;

  constructor(value: number) {
    this.value = [1];
  }
}

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

AnewValue

Bnode

Cvalue

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name not defined in the constructor.

Assigning 'val' or 'node' which are not parameters.

The constructor parameter is named 'value', so we assign it to this.value.

2fill in blank

medium

Complete the code to insert a new value into the BST recursively.

DSA Typescript

insert(node: Node | null, value: number): Node {
  if (node === null) {
    return new Node([1]);
  }
  if (value < node.value) {
    node.left = this.insert(node.left, value);
  } else {
    node.right = this.insert(node.right, value);
  }
  return node;
}

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

Athis

Bnode.value

Cnull

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'node.value' which is undefined when node is null.

Returning null instead of a new node.

When node is null, we create a new node with the value to insert.

3fill in blank

hard

Fix the error in the comparison to decide where to insert the new value.

DSA Typescript

if (value [1] node.value) {
  node.left = this.insert(node.left, value);
} else {
  node.right = this.insert(node.right, value);
}

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

A===

B<

C>

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' which reverses the BST property.

Using '===' or '!=' which are equality checks, not ordering.

In BST, if value is less than node.value, insert to the left subtree.

4fill in blank

hard

Fill both blanks to complete the insert method that updates the tree correctly.

DSA Typescript

insert(node: Node | null, value: number): Node {
  if (node === null) {
    return new Node([1]);
  }
  if (value [2] node.value) {
    node.left = this.insert(node.left, value);
  } else {
    node.right = this.insert(node.right, value);
  }
  return node;
}

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

Avalue

B<

C>

Dnode.value

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'node.value' in place of 'value' when creating new node.

Using '>' instead of '<' for left subtree insertion.

Create a new node with 'value' when node is null, and use '<' to decide left subtree insertion.

5fill in blank

hard

Fill all three blanks to complete the insert method with correct recursive calls and return.

DSA Typescript

insert(node: Node | null, value: number): Node {
  if (node === null) {
    return new Node([1]);
  }
  if (value [2] node.value) {
    node.left = this.insert(node.left, [3]);
  } else {
    node.right = this.insert(node.right, value);
  }
  return node;
}

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

Avalue

B<

Dnode.value

Attempts:

3 left

💡 Hint

Common Mistakes

Passing 'node.value' instead of 'value' in recursive call.

Using wrong comparison operator.

Not returning the updated node.

Create new node with 'value', use '<' for comparison, and pass 'value' recursively for left insertion.

Practice

1. What is the main purpose of the insert operation in a Binary Search Tree (BST)?

easy

A. To add a new node while keeping the BST property intact

B. To delete a node from the BST

C. To find the maximum value in the BST

D. To balance the BST after insertion

Solution

1. Step 1: Understand BST property

   A BST keeps nodes ordered so that left child is smaller and right child is larger than the parent.

2. Step 2: Purpose of insert operation

   Insert adds a new node in the correct position to maintain this order.

3. Final Answer:

   To add a new node while keeping the BST property intact -> Option A

4. Quick Check:

   Insert = Add node maintaining order [OK]

Hint: Insert adds nodes without breaking BST order [OK]

Common Mistakes:

• Confusing insert with delete operation
• Thinking insert balances the tree automatically
• Assuming insert finds max or min values

2. Which of the following is the correct TypeScript function signature for inserting a value into a BST node?

easy

A. function insert(root: Node, value: number): void

B. function insert(value: number, root: Node): void

C. function insert(root: Node | null, value: number): Node | null

D. function insert(root: Node, value: string): Node

Solution

1. Step 1: Analyze parameter types

   Insert needs the root node (which can be null if tree is empty) and the value to insert (a number).

2. Step 2: Analyze return type

   It returns the updated root node (Node or null if empty).

3. Final Answer:

   function insert(root: Node | null, value: number): Node | null -> Option C

4. Quick Check:

   Correct params and return type [OK]

Hint: Insert returns updated root, accepts root or null [OK]

Common Mistakes:

• Swapping parameter order
• Using void return type instead of returning Node
• Using wrong value type like string

3. Given the following BST and the insert operation call insert(root, 7), what will be the in-order traversal of the BST after insertion?

Initial BST in-order: 3 -> 5 -> 8 -> 10

Code snippet:

class Node {
  constructor(public val: number, public left: Node | null = null, public right: Node | null = null) {}
}

function insert(root: Node | null, val: number): Node {
  if (!root) return new Node(val);
  if (val < root.val) root.left = insert(root.left, val);
  else root.right = insert(root.right, val);
  return root;
}

medium

A. [3, 5, 8, 7, 10]

B. [3, 5, 8, 10, 7]

C. [7, 3, 5, 8, 10]

D. [3, 5, 7, 8, 10]

Solution

1. Step 1: Locate insertion point for 7

   7 is greater than 5 but less than 8, so it goes to the left of 8.

2. Step 2: Perform in-order traversal after insertion

   In-order visits left, root, right: 3, 5, 7, 8, 10.

3. Final Answer:

   [3, 5, 7, 8, 10] -> Option D

4. Quick Check:

   In-order sorted after insert [OK]

Hint: Insert left if smaller, right if larger [OK]

Common Mistakes:

• Placing 7 incorrectly after 8
• Confusing in-order traversal order
• Ignoring recursive insert logic

4. The following TypeScript code attempts to insert a value into a BST but has a bug. What is the error?

function insert(root: Node | null, val: number): Node {
  if (root === null) {
    return new Node(val);
  }
  if (val <= root.val) {
    insert(root.left, val);
  } else {
    insert(root.right, val);
  }
  return root;
}

medium

A. The base case is missing

B. The recursive calls do not update root.left or root.right

C. The function returns void instead of Node

D. The comparison operator should be < instead of <=

Solution

1. Step 1: Check recursive calls

   The calls to insert do not assign the result back to root.left or root.right, so tree structure is not updated.

2. Step 2: Correct recursive assignment

   Should be root.left = insert(root.left, val) or root.right = insert(root.right, val) to update links.

3. Final Answer:

   The recursive calls do not update root.left or root.right -> Option B

4. Quick Check:

   Must assign recursive insert result to child [OK]

Hint: Always assign recursive insert result to child node [OK]

Common Mistakes:

• Forgetting to assign recursive call result
• Misunderstanding base case presence
• Changing comparison operator unnecessarily

5. You have a BST with nodes 10, 5, 15, 3, 7, 12, 18. You want to insert 6. Which sequence of nodes will you visit during the insert operation?

hard

A. [10, 5, 7]

B. [10, 15, 12]

C. [10, 5, 3]

D. [10, 15, 18]

Solution

1. Step 1: Start at root 10

   6 is less than 10, so move to left child 5.

2. Step 2: Compare with 5 and 7

   6 is greater than 5, move to right child 7. 6 is less than 7, so insert here.

3. Final Answer:

   [10, 5, 7] -> Option A

4. Quick Check:

   Traverse nodes comparing 6 until insertion point [OK]

Hint: Follow left/right based on comparison until null [OK]

Common Mistakes:

• Visiting wrong subtree based on comparison
• Skipping nodes in path
• Confusing insertion path with traversal order