Binary Tree Node Structure in DSA Java - Build from Scratch

Binary Tree Node Structure

📖 Scenario: Imagine you are building a simple family tree app. Each person can have up to two children. To start, you need to create the basic building block: a node that holds a person's name and links to their children.

🎯 Goal: Build a Java class called TreeNode that represents a node in a binary tree. Each node should store a String called name and have two child nodes called left and right.

📋 What You'll Learn

Create a class named TreeNode

Add a String variable called name to store the person's name

Add two TreeNode variables called left and right for the children

Create a constructor that takes a String parameter to set the name

Initialize left and right to null in the constructor

💡 Why This Matters

🌍 Real World

Binary trees are used in many apps like family trees, decision trees, and organizing data hierarchically.

💼 Career

Understanding how to create and use tree nodes is essential for software developers working with data structures and algorithms.

1

Create the TreeNode class with name variable

Create a public class called TreeNode with a public String variable named name.

DSA Java

// Your code here

Hint

Start by writing public class TreeNode { and inside it declare public String name;

2

Add left and right child nodes

Inside the TreeNode class, add two public variables of type TreeNode named left and right.

DSA Java

public class TreeNode {
    public String name;
    // Your code here
}

Hint

Declare public TreeNode left; and public TreeNode right; inside the class.

3

Create constructor to set name and initialize children

Add a public constructor to TreeNode that takes a String parameter called name. Inside the constructor, set the class variable name to the parameter and initialize left and right to null.

DSA Java

public class TreeNode {
    public String name;
    public TreeNode left;
    public TreeNode right;

    // Your code here
}

Hint

Write a constructor public TreeNode(String name) and inside set this.name = name; and this.left = null; and this.right = null;.

4

Print the TreeNode name

Create a main method in a public class called Main. Inside main, create a TreeNode object named root with the name "Grandparent". Then print the name of root using System.out.println.

DSA Java

public class TreeNode {
    public String name;
    public TreeNode left;
    public TreeNode right;

    public TreeNode(String name) {
        this.name = name;
        this.left = null;
        this.right = null;
    }
}

// Your code here

Hint

Write a Main class with main method. Create TreeNode root = new TreeNode("Grandparent"); and print root.name.

Practice

1. What is the primary purpose of a TreeNode class in a binary tree?

easy

A. To perform tree traversal operations

B. To store only the root node of the tree

C. To store a value and references to left and right child nodes

D. To balance the binary tree automatically

Solution

1. Step 1: Understand the role of a node in a binary tree

   A node holds data and links to its children, enabling tree structure.

2. Step 2: Identify what the TreeNode class stores

   It stores a value and references (pointers) to left and right child nodes.

3. Final Answer:

   To store a value and references to left and right child nodes -> Option C

4. Quick Check:

   Node = value + left + right [OK]

Hint: Nodes hold data and links to children [OK]

Common Mistakes:

• Thinking TreeNode stores the whole tree
• Confusing node with traversal logic
• Assuming TreeNode balances the tree

2. Which of the following is the correct Java syntax to define a binary tree node class with integer data?

easy

A. class TreeNode { int data; TreeNode left; TreeNode right; }

B. class TreeNode { int data; TreeNode left; TreeNode right; TreeNode(int val) { data = val; } }

C. class TreeNode { int data; TreeNode left; TreeNode right; void TreeNode() {} }

D. class TreeNode { int data; TreeNode left, right; }

Solution

1. Step 1: Check class field declarations

   Fields for data, left, and right nodes must be declared properly.

2. Step 2: Verify constructor syntax

   Constructor should initialize data; class TreeNode { int data; TreeNode left; TreeNode right; TreeNode(int val) { data = val; } } correctly defines constructor.

3. Final Answer:

   class TreeNode { int data; TreeNode left; TreeNode right; TreeNode(int val) { data = val; } } -> Option B

4. Quick Check:

   Constructor initializes data correctly [OK]

Hint: Constructor must initialize data field [OK]

Common Mistakes:

• Missing constructor to set data
• Incorrect constructor syntax (void TreeNode())
• Declaring multiple variables incorrectly

3. Given the following code, what will be printed?

class TreeNode {
  int data;
  TreeNode left, right;
  TreeNode(int val) { data = val; }
}

public class Main {
  public static void main(String[] args) {
    TreeNode root = new TreeNode(10);
    root.left = new TreeNode(5);
    root.right = new TreeNode(15);
    System.out.println(root.left.data + "," + root.right.data);
  }
}

medium

A. 5,15

B. 10,5

C. 15,5

D. 10,15

Solution

1. Step 1: Create root node with data 10

   Root node data is 10, left and right are null initially.

2. Step 2: Assign left and right children with data 5 and 15

   root.left.data = 5, root.right.data = 15.

3. Step 3: Print left and right data

   Output is "5,15" from root.left.data and root.right.data.

4. Final Answer:

   5,15 -> Option A

5. Quick Check:

   Left=5, Right=15 [OK]

Hint: Print left and right node data directly [OK]

Common Mistakes:

• Printing root data instead of children
• Swapping left and right values
• Confusing node references

4. Identify the error in the following TreeNode class code:

class TreeNode {
  int data;
  TreeNode left, right;
  void TreeNode(int val) {
    data = val;
  }
}

medium

A. Constructor has void return type, so it's a method not a constructor

B. Missing semicolon after data declaration

C. left and right should be arrays, not single nodes

D. Class name and constructor name do not match

Solution

1. Step 1: Check constructor syntax

   Constructor should not have a return type; 'void' makes it a method.

2. Step 2: Confirm class and constructor names match

   Names match, so no error there.

3. Final Answer:

   Constructor has void return type, so it's a method not a constructor -> Option A

4. Quick Check:

   Constructor no return type [OK]

Hint: Constructor must not have a return type [OK]

Common Mistakes:

• Adding void before constructor name
• Confusing methods with constructors
• Thinking left/right must be arrays

5. You want to create a binary tree node class that also stores a parent node reference. Which of the following class definitions correctly adds this feature?

hard

A. class TreeNode { int data; TreeNode left, right, parent; TreeNode(int val) { data = val; } }

B. class TreeNode { int data; TreeNode left, right; TreeNode parent; TreeNode(int val) { data = val; parent = null; } }

C. class TreeNode { int data; TreeNode left, right; TreeNode parent; TreeNode(int val) { data = val; left = right = parent = null; } }

D. class TreeNode { int data; TreeNode left, right; TreeNode parent; TreeNode(int val, TreeNode p) { data = val; parent = p; } }

Solution

1. Step 1: Understand the need for parent reference initialization

   Parent reference should be set when node is created to link properly.

2. Step 2: Check constructor parameters and assignments

   class TreeNode { int data; TreeNode left, right; TreeNode parent; TreeNode(int val, TreeNode p) { data = val; parent = p; } } passes parent node as parameter and assigns it correctly.

3. Final Answer:

   class TreeNode { int data; TreeNode left, right; TreeNode parent; TreeNode(int val, TreeNode p) { data = val; parent = p; } } -> Option D

4. Quick Check:

   Parent passed and assigned in constructor [OK]

Hint: Pass parent node in constructor to set reference [OK]

Common Mistakes:

• Not initializing parent reference
• Setting parent to null always
• Declaring parent but never assigning