Binary Tree Node Structure in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a binary tree node?

A binary tree node is a basic unit of a binary tree that contains data and references to two child nodes: left and right.

beginner

What are the main parts of a binary tree node in C#?

A binary tree node in C# typically has three parts:
1. Data (value stored)
2. Left child reference
3. Right child reference

beginner

Show a simple C# class definition for a binary tree node.

public class TreeNode<br>{<br>  public int Value;<br>  public TreeNode Left;<br>  public TreeNode Right;<br>  public TreeNode(int value)<br>  {<br>    Value = value;<br>    Left = null;<br>    Right = null;<br>  }<br>}

beginner

Why do binary tree nodes have left and right references?

Left and right references point to the node's children, allowing the tree to branch into two directions, which helps organize data hierarchically.

beginner

What does it mean if a binary tree node's left or right reference is null?

It means that the node does not have a child on that side, indicating the end of that branch in the tree.

What does a binary tree node typically store?

A Data and three child references

B Only data

C Only child references

D Data and two child references

A binary tree node stores data and references to two children: left and right.

In C#, how do you represent the left child of a binary tree node?

A public Node LeftChild;

B public TreeNode Left;

C public TreeNode ChildLeft;

D public int Left;

The left child is represented as a reference to another TreeNode object, typically named Left.

What does it mean if a node's Right property is null?

A The node has no right child

B The node has a right child

C The node is the root

D The node is a leaf

A null Right property means no right child exists for that node.

Which of these is NOT part of a binary tree node?

A Parent reference

B Data value

C Right child reference

D Left child reference

A basic binary tree node does not usually store a parent reference.

What is the purpose of the constructor in the TreeNode class?

A To print the node's value

B To delete the node

C To initialize the node's value and set children to null

D To connect the node to its parent

The constructor sets the node's value and initializes left and right children to null.

Describe the structure of a binary tree node and its components.

Think about what each node holds and how it connects to others.

Write a simple C# class for a binary tree node including a constructor.

Focus on fields and constructor setup.

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