Binary Tree Node Structure in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Binary Tree Node Creation

What is the output of the following C# code that creates a simple binary tree node and prints its value?

DSA C#

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

TreeNode root = new TreeNode(10);
Console.WriteLine(root.val);

A null

B 0

C 10

D Compilation error

💡 Hint

Check the value assigned to the root node and what is printed.

The TreeNode constructor sets val to 10. Printing root.val outputs 10.

❓ Predict Output

intermediate

Value of Left Child Node

Given the following code, what is the value printed for the left child node?

DSA C#

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

TreeNode root = new TreeNode(5);
root.left = new TreeNode(3);
Console.WriteLine(root.left.val);

A 3

B 5

C null

D Compilation error

💡 Hint

Look at the value assigned to root.left.

The left child node is created with value 3, so printing root.left.val outputs 3.

🔧 Debug

advanced

Identify the Error in TreeNode Initialization

What error will this code produce when compiled and run?

DSA C#

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

TreeNode root = new TreeNode();

A Compilation error: no argument given for constructor

B Runtime NullReferenceException

C Outputs 0

D No error, runs fine

💡 Hint

Check the constructor parameters and how the object is created.

The TreeNode class has a constructor that requires an int argument. Calling new TreeNode() without arguments causes a compilation error.

🧠 Conceptual

advanced

Binary Tree Node Left and Right Initialization

In a binary tree node class, what is the default value of the left and right child nodes if they are not explicitly assigned?

A Undefined, causes runtime error

B 0

C A new TreeNode with value 0

D null

💡 Hint

Consider what happens to reference type fields when not assigned.

In C#, reference type fields like left and right default to null if not assigned.

❓ Predict Output

expert

Output After Multiple Node Assignments

What is the output of the following code snippet?

DSA C#

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

TreeNode root = new TreeNode(1);
root.left = new TreeNode(2);
root.right = new TreeNode(3);
root.left.left = new TreeNode(4);
root.left.right = new TreeNode(5);

Console.WriteLine(root.left.right.val);

A 4

B 5

C 3

D Compilation error

💡 Hint

Trace the assignments from root to left child to right child.

root.left.right is assigned a TreeNode with value 5, so printing its val outputs 5.

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