BST Insert Operation in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA Python

class Node:
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = None

new_node = Node([1])

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

Aval

B5

Cvalue

Dnode

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a variable name instead of a value.

Using an undefined variable.

The new node is created with the value 5 passed as an argument.

2fill in blank

medium

Complete the code to check if the current node is empty before inserting.

DSA Python

def insert(root, val):
    if root is [1]:
        return Node(val)

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

Aempty

BFalse

CTrue

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'False' instead of 'None'.

Using 'empty' which is not a Python keyword.

We check if the root is None to know if the tree or subtree is empty.

3fill in blank

hard

Fix the error in the recursive call to insert the value in the left subtree.

DSA Python

def insert(root, val):
    if val < root.val:
        root.left = insert(root.[1], val)
    return root

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

Aleft

Bval

Cright

Droot

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'right' instead of 'left'.

Using 'val' or 'root' which are incorrect attributes.

To insert in the left subtree, we recurse on root.left.

4fill in blank

hard

Fill both blanks to insert the value in the right subtree when val is greater or equal.

DSA Python

def insert(root, val):
    if val >= root.val:
        root.[1] = insert(root.[2], val)
    return root

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

Aright

Bleft

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'left' for either blank.

Using 'val' which is not a child attribute.

For values greater or equal, insert into the right subtree using root.right.

5fill in blank

hard

Fill all three blanks to complete the BST insert function with base case and recursive calls.

DSA Python

def insert(root, val):
    if root is [1]:
        return Node(val)
    if val < root.val:
        root.left = insert(root.left, [2])
    else:
        root.right = insert(root.right, [3])
    return root

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

ANone

Bval

Droot

Attempts:

3 left

💡 Hint

Common Mistakes

Passing root instead of val in recursive calls.

Using incorrect base case check.

The base case checks if root is None. The recursive calls pass the value to insert.

Practice

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

easy

A. To balance the BST automatically

B. To add a new value while keeping the BST property intact

C. To find the maximum value in the BST

D. To delete a node from the BST

Solution

1. Step 1: Understand the BST property

   A BST keeps values in order: left child smaller, right child larger.

2. Step 2: Purpose of insert operation

   Insert adds a new value so the tree remains ordered by BST rules.

3. Final Answer:

   To add a new value while keeping the BST property intact -> Option B

4. Quick Check:

   Insert = Add value maintaining order [OK]

Hint: Insert means add value keeping order [OK]

Common Mistakes:

• Confusing insert with delete
• Thinking insert balances the tree automatically
• Believing insert finds max value

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

easy

A. def insert(root, value):

B. def insert(value, root):

C. def insert(value):

D. def insert(root):

Solution

1. Step 1: Identify parameters needed for insertion

   We need the root node to start and the value to insert.

2. Step 2: Check function signature correctness

   Signature with root first, then value is common and clear: def insert(root, value):.

3. Final Answer:

   def insert(root, value): -> Option A

4. Quick Check:

   Insert(root, value) signature [OK]

Hint: Insert needs root node and value [OK]

Common Mistakes:

• Swapping parameter order
• Missing root parameter
• Using only one parameter

3. What will be the output of the BST after inserting 7 into the BST shown below?

Initial BST (inorder): 3 -> 5 -> 8 -> 10 -> null

Code snippet:

root = BSTNode(5)
root.left = BSTNode(3)
root.right = BSTNode(8)
root.right.right = BSTNode(10)
root = insert(root, 7)
print_inorder(root)

medium

A. 3 -> 5 -> 7 -> 8 -> 10 -> null

B. 3 -> 5 -> 8 -> 7 -> 10 -> null

C. 3 -> 5 -> 8 -> 10 -> 7 -> null

D. 3 -> 5 -> 7 -> 10 -> 8 -> null

Solution

1. Step 1: Locate correct position for 7 in BST

   7 is greater than 5, so go right to 8. 7 is less than 8, so insert as left child of 8.

2. Step 2: Perform inorder traversal after insertion

   Inorder visits left, root, right: 3, 5, 7, 8, 10.

3. Final Answer:

   3 -> 5 -> 7 -> 8 -> 10 -> null -> Option A

4. Quick Check:

   Inorder after insert 7 = 3,5,7,8,10 [OK]

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

Common Mistakes:

• Inserting 7 as right child of 8
• Not updating tree links correctly
• Confusing inorder output order

4. Consider this buggy insert function for BST:

def insert(root, val):
    if root is None:
        return BSTNode(val)
    if val < root.val:
        insert(root.left, val)
    else:
        insert(root.right, val)
    return root

What is the main problem with this code?

medium

A. It uses wrong comparison operators

B. It does not return the new root node

C. It inserts duplicates incorrectly

D. It does not assign the recursive insert result to child nodes

Solution

1. Step 1: Check recursive calls and assignments

   The recursive calls to insert do not assign the returned node back to root.left or root.right.

2. Step 2: Understand why assignment is needed

   Without assignment, new nodes are created but not linked to the tree, so insertion fails.

3. Final Answer:

   It does not assign the recursive insert result to child nodes -> Option D

4. Quick Check:

   Assign recursive insert to children [OK]

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

Common Mistakes:

• Ignoring return value of recursive insert
• Assuming insert auto-links nodes
• Confusing comparison operators

5. You have a BST with nodes: 15, 10, 20, 8, 12, 17, 25. You want to insert 19. After insertion, what is the inorder traversal of the BST?

hard

A. 8 -> 10 -> 12 -> 15 -> 19 -> 17 -> 20 -> 25 -> null

B. 8 -> 10 -> 12 -> 15 -> 17 -> 20 -> 19 -> 25 -> null

C. 8 -> 10 -> 12 -> 15 -> 17 -> 19 -> 20 -> 25 -> null

D. 8 -> 10 -> 12 -> 15 -> 17 -> 20 -> 25 -> 19 -> null

Solution

1. Step 1: Find correct position for 19 in BST

   Start at root 15: 19 > 15, go right to 20. 19 < 20, go left to 17. 19 > 17, insert as right child of 17.

2. Step 2: Perform inorder traversal after insertion

   Inorder visits nodes in ascending order: 8, 10, 12, 15, 17, 19, 20, 25.

3. Final Answer:

   8 -> 10 -> 12 -> 15 -> 17 -> 19 -> 20 -> 25 -> null -> Option C

4. Quick Check:

   Inorder after insert 19 = sorted nodes [OK]

Hint: Insert by comparing down tree, then inorder print [OK]

Common Mistakes:

• Inserting 19 as left child of 20 without checking 17
• Misordering nodes in inorder traversal
• Confusing left and right child positions