Whats wrong in this BST implementation?

Question

Why am I getting false while doing search(7) ? I tried recursive solution its working fine.. tried implementing with loop failed

public class BST {

    class Node {
        int data;
        Node left , right;

        public Node(int data) {
            this.data = data;
            this.left = this.right = null;
        }
    }

    Node root;

    public BST() {
        this.root = null;
    }

    public void insert(int data) {
        // create a new node and start iteration from root node
        Node newNode = new Node(data);
        Node currentNode = this.root;

        while (true) {
            if (currentNode == null) {
                currentNode = newNode;
                break;
            }

            if (data < currentNode.data) { // if data is less go left
                currentNode = currentNode.left;
            } else if (data > currentNode.data) {   // if data is greater go right
                currentNode = currentNode.right;
            } else {    // do nothing for duplicates
                break;
            }
        }
    }

    public boolean search(int data) {
        Node currentNode = this.root;

        while (true) {
            if (currentNode == null) {
                return false;
            }

            if (data == currentNode.data) {
                return true;
            } else if (data < currentNode.data) {
                currentNode = currentNode.left;
            } else {
                currentNode = currentNode.right;
            }
        }
    }

    public static void main(String... args) {
        BST tree = new BST();

        tree.insert(15);
        tree.insert(7);

        System.out.println(tree.search(7));
    }
}

Answer 1

Problem is inside the insert method - you are not inserting the nodes correctly inside the tree.

1. If the tree is empty, you should assign to this.root , not currentNode . Assigning to currentNode has no affect on this.root .

   Currently, your code isn't inserting any node inside the tree; it simply assigns the new node to the local variable of insert method, ie currentNode .

2. If the condition data < currentNode.data is true, then you need to check if the currentNode.left is null or not. If it is, then link the new node with the current node as shown below:

    currentNode.left = newNode;

   If currentNode.left is not null , then do the following:

    currentNode = currentNode.left;

   Currently, your code moves the currentNode to null and then assigns the newNode to the currentNode without a reference to its parent node in the tree .

3. Do step 2 for data > currentNode.data as well.

Change the implementation of the insert method as shown below:

public void insert(int data) {

    Node newNode = new Node(data);
    
    if (this.root == null) {
        this.root = newNode;
        return;
    }
    
    Node currentNode = this.root;

    while (true) {
        if (data < currentNode.data) {
            if (currentNode.left == null) {
                currentNode.left = newNode;
            } else {
                currentNode = currentNode.left;
            }
        } else if (data > currentNode.data) {
            if (currentNode.right == null) {
                currentNode.right = newNode;
            } else {
                currentNode = currentNode.right;
            }
        } else {
            break;
        }
    }
}