Binary Tree Node Structure in DSA Java - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Simple Binary Tree Node Creation

What is the printed output after creating a binary tree node with value 10 and printing its value and children?

DSA Java

class Node {
    int val;
    Node left, right;
    Node(int val) {
        this.val = val;
        this.left = null;
        this.right = null;
    }
}

public class Main {
    public static void main(String[] args) {
        Node root = new Node(10);
        System.out.print(root.val + " -> ");
        System.out.print(root.left + " -> ");
        System.out.print(root.right);
    }
}

A 10 -> null -> null

B 10 -> 0 -> 0

C 10 -> ->

D 10 -> Node@15db9742 -> Node@6d06d69c

💡 Hint

Remember that uninitialized object references in Java default to null.

The node is created with value 10 and both left and right children are null. Printing null references shows 'null'.

❓ Predict Output

intermediate

Output After Linking Left Child Node

What is the output after creating a root node with value 5 and linking a left child node with value 3, then printing the values?

DSA Java

class Node {
    int val;
    Node left, right;
    Node(int val) {
        this.val = val;
        this.left = null;
        this.right = null;
    }
}

public class Main {
    public static void main(String[] args) {
        Node root = new Node(5);
        root.left = new Node(3);
        System.out.print(root.val + " -> ");
        System.out.print(root.left.val + " -> ");
        System.out.print(root.right);
    }
}

A 5 -> Node@15db9742 -> null

B 5 -> null -> null

C 5 -> 3 -> null

D 5 -> 3 -> 0

💡 Hint

Check what happens when you print a null reference versus an object.

The root has value 5, left child has value 3, right child is null. Printing left.val prints 3, right prints null.

🔧 Debug

advanced

Identify the Error in Binary Tree Node Initialization

What error will this code produce when trying to compile and run?

DSA Java

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

public class Main {
    public static void main(String[] args) {
        Node root = new Node(1);
        System.out.println(root.val);
    }
}

A TypeError: incompatible types

B NullPointerException at runtime

C No error, prints 1

D SyntaxError: missing semicolon

💡 Hint

Check the line where left and right are assigned.

The line 'left = right = null' is missing a semicolon at the end, causing a syntax error.

🧠 Conceptual

advanced

Understanding Null Children in Binary Tree Nodes

Why do we initialize left and right child nodes to null in a binary tree node?

A To save memory by avoiding object creation

B To indicate the node currently has no children

C To automatically create child nodes later

D To prevent the node from being garbage collected

💡 Hint

Think about what null means for references in Java.

Null means the node does not point to any child node yet, showing the tree structure clearly.

🚀 Application

expert

Resulting Tree Structure After Multiple Insertions

Given the following code that creates nodes and links them, what is the printed output representing the tree nodes in preorder (root -> left -> right)?

DSA Java

class Node {
    int val;
    Node left, right;
    Node(int val) {
        this.val = val;
        this.left = null;
        this.right = null;
    }
}

public class Main {
    public static void preorder(Node node) {
        if (node == null) return;
        System.out.print(node.val + " -> ");
        preorder(node.left);
        preorder(node.right);
    }

    public static void main(String[] args) {
        Node root = new Node(10);
        root.left = new Node(5);
        root.right = new Node(15);
        root.left.left = new Node(3);
        root.left.right = new Node(7);
        preorder(root);
    }
}

A 10 -> 5 -> 3 -> 7 -> 15 ->

B 10 -> 5 -> 7 -> 3 -> 15 ->

C 3 -> 5 -> 7 -> 10 -> 15 ->

D 10 -> 15 -> 5 -> 7 -> 3 ->

💡 Hint

Preorder traversal visits root first, then left subtree, then right subtree.

The traversal order is root(10), left(5), left-left(3), left-right(7), right(15).

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