Binary Tree Node Structure in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the data field in a binary tree node class.

DSA C#

public class TreeNode {
    public int [1];
    public TreeNode left, right;

    public TreeNode(int value) {
        this.value = value;
        left = right = null;
    }
}

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

Avalue

Bnode

Cval

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'data' or 'val' instead of 'value' causes mismatch with constructor.

Using 'node' is incorrect as it is not a value field.

The field storing the node's value is named 'value' as used in the constructor and class.

2fill in blank

medium

Complete the code to initialize the left child pointer in the constructor.

DSA C#

public TreeNode(int value) {
    this.value = value;
    this.left = [1];
    this.right = null;
}

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

Anew TreeNode()

Bthis

Cvalue

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning 'this' causes self-reference errors.

Using 'new TreeNode()' requires parameters and is unnecessary.

The left child pointer is initialized to null to indicate no child initially.

3fill in blank

hard

Fix the error in the constructor to correctly assign the right child pointer.

DSA C#

public TreeNode(int value) {
    this.value = value;
    left = null;
    right = [1];
}

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

Anull

Bnew TreeNode()

Cvalue

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning 'this' causes circular reference.

Assigning 'value' is incorrect as it's an int, not a node.

The right child pointer should be set to null initially, not to 'this' or other values.

4fill in blank

hard

Fill both blanks to declare and initialize a binary tree node's value and left child.

DSA C#

public class TreeNode {
    public int [1];
    public TreeNode [2];

    public TreeNode(int value) {
        this.value = value;
        left = null;
    }
}

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

Avalue

Bdata

Cleft

Dright

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up left and right child names.

Using 'data' instead of 'value' for the integer field.

The value field stores the node's data, and left is the left child pointer.

5fill in blank

hard

Fill all three blanks to declare a binary tree node with value, left, and right pointers.

DSA C#

public class TreeNode {
    public int [1];
    public TreeNode [2], [3];

    public TreeNode(int value) {
        this.value = value;
        left = right = null;
    }
}

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

Avalue

Bleft

Cright

Dnode

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'node' instead of 'left' or 'right' for child pointers.

Swapping left and right names.

The node has an integer 'value' and two child pointers 'left' and 'right'.

Practice

1. What does a typical binary tree node contain in its structure?

easy

A. Only a value without any references

B. A value and a reference to the next node in a list

C. References to parent and sibling nodes only

D. A value and references to left and right child nodes

Solution

1. Step 1: Understand binary tree node components

   A binary tree node stores data and links to its children nodes.

2. Step 2: Identify correct node structure

   It must have a value and two references: one for left child and one for right child.

3. Final Answer:

   A value and references to left and right child nodes -> Option D

4. Quick Check:

   Binary tree node = value + left + right [OK]

Hint: Remember: binary tree nodes have two child links [OK]

Common Mistakes:

• Thinking nodes only store values without links
• Confusing with linked list node structure
• Assuming nodes have parent or sibling references by default

2. Which of the following is the correct C# syntax to declare a binary tree node class with integer value and left/right children?

easy

A. public struct Node { int val; Node left; Node right; }

B. class Node { int val; Node left, right; }

C. public class Node { public int val; public Node left; public Node right; }

D. public class Node { int val; Node left; Node right; void Node() {} }

Solution

1. Step 1: Check access modifiers and field declarations

   Public class with public fields allows access outside the class, correct for node structure.

2. Step 2: Validate syntax correctness

   public class Node { public int val; public Node left; public Node right; } uses public class, public int val, and public Node left/right fields correctly.

3. Final Answer:

   public class Node { public int val; public Node left; public Node right; } -> Option C

4. Quick Check:

   Public class with public fields = public class Node { public int val; public Node left; public Node right; } [OK]

Hint: Use public class and public fields for node structure [OK]

Common Mistakes:

• Missing public keyword causing access issues
• Using struct instead of class for nodes
• Incorrect constructor syntax or missing access modifiers

3. Given the following code, what will be the output when printing the root node's left child's value?

public class Node {
  public int val;
  public Node left;
  public Node right;
  public Node(int v) { val = v; }
}

Node root = new Node(10);
root.left = new Node(5);
root.right = new Node(15);
Console.WriteLine(root.left.val);

medium

A. 5

B. null

C. 15

D. 10

Solution

1. Step 1: Trace node assignments

   Root node value is 10, left child assigned new Node with value 5.

2. Step 2: Identify printed value

   Printing root.left.val outputs 5, the left child's value.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   root.left.val = 5 [OK]

Hint: Print left child's val to get 5 [OK]

Common Mistakes:

• Printing root.val instead of root.left.val
• Confusing left and right child values
• Assuming uninitialized children print null

4. Identify the error in the following binary tree node class code:

public class Node {
  public int val;
  public Node left, right;
  public Node(int value) {
    val = value;
    left = null;
    right = null;
  }
}

Node root = new Node(10);
root.left.val = 5;

medium

A. Cannot assign val to left child before initializing left node

B. Constructor missing return type

C. left and right should be private fields

D. Node class should be a struct

Solution

1. Step 1: Analyze node initialization

   left and right are initialized to null in constructor, so root.left is null initially.

2. Step 2: Check assignment to left.val

   Trying to assign root.left.val = 5 causes null reference error because root.left is null.

3. Final Answer:

   Cannot assign val to left child before initializing left node -> Option A

4. Quick Check:

   Assigning to null child causes error [OK]

Hint: Initialize child node before assigning its value [OK]

Common Mistakes:

• Assuming child nodes auto-initialize
• Ignoring null reference exceptions
• Confusing constructor syntax errors

5. How would you modify the binary tree node class to include a method that returns true if the node is a leaf (has no children)?

hard

A. public bool IsLeaf() { return val == null; }

B. public bool IsLeaf() { return left == null && right == null; }

C. public bool IsLeaf() { return left != null || right != null; }

D. public bool IsLeaf() { return left == right; }

Solution

1. Step 1: Define leaf node condition

   A leaf node has no left or right children, so both must be null.

2. Step 2: Write method to check children

   Return true if left and right are null, false otherwise.

3. Final Answer:

   public bool IsLeaf() { return left == null && right == null; } -> Option B

4. Quick Check:

   Leaf check = left and right null [OK]

Hint: Leaf means no children: left and right null [OK]

Common Mistakes:

• Checking value instead of children
• Using OR instead of AND in condition
• Comparing left and right references directly