BST Insert Operation in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to insert a new node in the BST when the tree is empty.

DSA C#

if (root == null)
{
    root = new Node([1]);
    return root;
}

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

Avalue

Broot

Cnew Node()

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'root' instead of 'value' inside the new Node constructor.

Assigning null instead of creating a new node.

When the tree is empty, we create a new node with the given value and assign it as the root.

2fill in blank

medium

Complete the code to decide to insert the new value in the left subtree.

DSA C#

if (value [1] root.Data)
{
    root.Left = Insert(root.Left, value);
}

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

A<

B>

C==

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' causes wrong subtree insertion.

Using '==' which is for equality, not ordering.

If the value to insert is less than the current node's data, we insert it into the left subtree.

3fill in blank

hard

Fix the error in the recursive call to insert the new value in the right subtree.

DSA C#

else if (value [1] root.Data)
{
    root.Right = Insert(root.Right, value);
}

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

A<

B>

C<=

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' causes insertion in the wrong subtree.

Using '==' which is for equality, not ordering.

If the value is greater than the current node's data, insert it into the right subtree.

4fill in blank

hard

Fill both blanks to complete the Insert method that returns the updated root after insertion.

DSA C#

public Node Insert(Node root, int value)
{
    if (root == null)
        root = new Node([1]);
    else if (value [2] root.Data)
        root.Left = Insert(root.Left, value);
    else
        root.Right = Insert(root.Right, value);
    return root;
}

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

Avalue

B<

C>

Droot

Attempts:

3 left

💡 Hint

Common Mistakes

Using root instead of value when creating new node.

Using wrong comparison operator for left subtree.

The method creates a new node with the value if root is null, then inserts recursively to left if value is less than root data.

5fill in blank

hard

Fill all three blanks to complete the Insert method with correct comparisons and return.

DSA C#

public Node Insert(Node root, int value)
{
    if (root == null)
        root = new Node([1]);
    else if (value [2] root.Data)
        root.Left = Insert(root.Left, value);
    else if (value [3] root.Data)
        root.Right = Insert(root.Right, value);
    return root;
}

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

Avalue

B<

C>

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '<' or '>' for subtree decisions.

Not returning the root after insertion.

The Insert method creates a new node with the value if root is null, inserts to left if value is less, to right if value is greater, and returns the root.

Practice

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

easy

A. To delete a node from the BST

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

C. To find the maximum value in the BST

D. To balance the BST automatically

Solution

1. Step 1: Understand the Insert operation

   The Insert operation adds a new value to the BST while maintaining the order property: left child < parent < right child.

2. Step 2: Identify the purpose of Insert

   Insert does not delete nodes, find max, or balance the tree automatically. Its main job is to add nodes correctly.

3. Final Answer:

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

4. Quick Check:

   Insert = Add node correctly [OK]

Hint: Insert adds nodes preserving 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 C# method signature for inserting a value into a BST node class?

easy

A. public void Insert(int value)

B. void InsertNode(int value, Node root)

C. private Node Insert(int value, Node root)

D. public int Insert(int value)

Solution

1. Step 1: Understand typical Insert method signature

   Insert usually returns the updated root node after insertion, so return type is Node, and it takes the root and value as parameters.

2. Step 2: Check method access and parameters

   Private method with signature Node Insert(int value, Node root) fits common recursive insert pattern.

3. Final Answer:

   private Node Insert(int value, Node root) -> Option C

4. Quick Check:

   Insert returns updated node [OK]

Hint: Insert returns updated root node [OK]

Common Mistakes:

• Using void return type for recursive insert
• Incorrect parameter order or missing root node
• Making Insert public when it should be private helper

3. What will be the in-order traversal output after inserting 15 into this BST?

      10
     /  \
    5    20
         /
        17

Insert operation: Insert(15)

medium

A. 5 -> 10 -> 15 -> 17 -> 20 -> null

B. 5 -> 10 -> 17 -> 15 -> 20 -> null

C. 5 -> 10 -> 15 -> 20 -> 17 -> null

D. 5 -> 10 -> 20 -> 17 -> 15 -> null

Solution

1. Step 1: Insert 15 into BST

   Starting at root 10, 15 > 10, go right to 20. 15 < 20, go left to 17. 15 < 17, insert as left child of 17.

2. Step 2: Perform in-order traversal

   In-order visits left, root, right: 5, 10, 15, 17, 20.

3. Final Answer:

   5 -> 10 -> 15 -> 17 -> 20 -> null -> Option A

4. Quick Check:

   In-order sorted = 5,10,15,17,20 [OK]

Hint: Insert left if smaller, right if larger, then in-order sorts [OK]

Common Mistakes:

• Inserting 15 as right child of 17
• Misordering nodes in traversal
• Ignoring BST property during insertion

4. Given this Insert method snippet, what is the error?

private Node Insert(int value, Node root) {
  if (root == null) {
    root = new Node(value);
  } else if (value < root.Value) {
    Insert(value, root.Left);
  } else {
    Insert(value, root.Right);
  }
  return root;
}

medium

A. Missing assignment when inserting into left or right subtree

B. Incorrect base case for null root

C. Method should return void instead of Node

D. Using else instead of else if for right subtree insertion

Solution

1. Step 1: Analyze recursive calls

   Recursive calls to Insert on left or right subtree do not assign the returned node back to root.Left or root.Right.

2. Step 2: Identify fix

   Must assign: root.Left = Insert(value, root.Left); and similarly for right to update tree links.

3. Final Answer:

   Missing assignment when inserting into left or right subtree -> Option A

4. Quick Check:

   Assign recursive result to subtree [OK]

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

Common Mistakes:

• Not assigning recursive insert result
• Changing base case incorrectly
• Confusing return type with void

5. You have a BST with nodes 10, 5, 15. You want to insert 12 and 18. After insertion, which of the following represents the correct pre-order traversal?

hard

A. 10 -> 15 -> 18 -> 12 -> 5 -> null

B. 10 -> 5 -> 12 -> 15 -> 18 -> null

C. 10 -> 15 -> 5 -> 12 -> 18 -> null

D. 10 -> 5 -> 15 -> 12 -> 18 -> null

Solution

1. Step 1: Insert 12 and 18 into BST

   Start with root 10, left child 5, right child 15. Insert 12: 12 < 15, goes left of 15. Insert 18: 18 > 15, goes right of 15.

2. Step 2: Perform pre-order traversal

   Pre-order visits root, left, right: 10, 5, 15, 12, 18.

3. Final Answer:

   10 -> 5 -> 15 -> 12 -> 18 -> null -> Option D

4. Quick Check:

   Pre-order = root-left-right [OK]

Hint: Pre-order: root first, then left, then right [OK]

Common Mistakes:

• Mixing pre-order with in-order or post-order
• Inserting nodes in wrong subtree
• Misplacing 12 and 18 in traversal