How to Implement Binary Search Tree in Java? Example

A binary search tree or BST is a popular data structure that is used to keep elements in order. A binary search tree is a binary tree where the value of a left child is less than or equal to the parent node and the value of the right child is greater than or equal to the parent node. Since it's a binary tree, it can only have 0, 1, or two children. What makes a binary search tree special is its ability to reduce the time complexity of fundamental operations like add, remove, and search, also known as insert, delete and find. In a BST, all these operations (insert, remove, and find) can be performed in O(log(n)) time.

The reason for this improvement in speed is because of the unique property of the binary search tree, where for each node, the data in the left child is less than (or equal) and the data in the right child is greater than (or equal) to the data in the said node.

In Programming interviews, you will see many data structures and algorithmic questions based upon a binary search tree e.g. check if a binary tree is a BST or not? Or, write a program to check if BST is balanced or not. In order to solve that problem, you must know how to implement BST in Java.

In this tutorial, I will teach you how to implement a binary search tree in Java, which you can use to solve any binary search tree or binary tree-based coding problems.

Also, basic knowledge of essential data structure is also very important and that's why I suggest all Java programmers join a comprehensive Data Structure and Algorithms course like Data Structures and Algorithms: Deep Dive Using Java on Udemy to fill the gaps in your understanding.

Binary Search tree in Java

Here, You will learn how to create a binary search tree with integer nodes. I am not using Generics just to keep the code simple but if you like you can extend the problem to use Generics, which will allow you to create a Binary tree of String, Integer, Float, or Double. Remember, you make sure that node of BST must implement the Comparable interface. This is what many Java programmers forget when they try to implement binary search trees with Generics.

Here is an implementation of a binary search tree in Java. It's just a structure, we will subsequently add methods to add a node in a binary search tree, delete a node from the binary search tree and find a node from BST in the subsequent part of this binary search tree tutorial.

In this implementation, I have created a Node class, which is similar to our linked list node class, which we created when I have shown you how to implement a linked list in Java. It has a data element, an integer, and a Node reference to point to another node in the binary tree.

I have also created four basic functions, as shown below:
•  getRoot(), returns the root of binary tree
•  isEmpty(), to check if the binary search tree is empty or not
•  size(), to find the total number of nodes in a BST
•  clear(), to clear the BST

For a curious developer who wants to learn advanced data structure in Java, I also recommend checking out Data Structures and Algorithms in Java, 2nd edition, one of the rare books where you will find examples in Java.

Here is the sample code to create a binary search tree or BST in Java, without using any third-party library.

Java Program to represent Binary Search Tree or BST

```import java.util.Stack;

/**
* Java Program to implement a binary search tree. A binary search tree is a
* sorted binary tree, where value of a node is greater than or equal to its
* left the child and less than or equal to its right child.
*
* @author WINDOWS 8
*
*/
public class BST {

private static class Node {
private int data;
private Node left, right;

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

private Node root;

public BST() {
root = null;
}

public Node getRoot() {
return root;
}

/**
* Java function to check if binary tree is empty or not
* Time Complexity of this solution is constant O(1) for
* best, average and worst case.
*
* @return true if binary search tree is empty
*/
public boolean isEmpty() {
return null == root;
}

/**
* Java function to return number of nodes in this binary search tree.
* Time complexity of this method is O(n)
* @return size of this binary search tree
*/
public int size() {
Node current = root;
int size = 0;
Stack<Node> stack = new Stack<Node>();
while (!stack.isEmpty() || current != null) {
if (current != null) {
stack.push(current);
current = current.left;
} else {
size++;
current = stack.pop();
current = current.right;
}
}
return size;
}

/**
* Java function to clear the binary search tree.
* Time complexity of this method is O(1)
*/
public void clear() {
root = null;
}

}```

That's all in this tutorial about how to implement a binary search tree in Java. In this tutorial, you have learned to create the structure of BST using Node class and some basic functions. In the next couple of tutorials, you will learn some more interesting things with BST like writing a method to add Nodes in BST, this method must make sure that the property of the binary search tree is not violated. I mean, it first needs to find the right place and then needs to add the element. Subsequently, you will also learn how to search a node in the binary search tree.

If you are interested in learning Data structure and Algorithms in the Java Programming language then you can following books that have several examples of the tree, linked list, heap, and other advanced data structures in Java.

• 5 Data Structure and Algorithm Books for Java Developers (list)
• 30 Array-based questions from Programming interviews (article)
• Sieve of Eratosthenes algorithms for Prime numbers (algorithm)
• How to reverse an array in place? (solution)
• 20 String based questions from Programming Job Interviews (article)
• How to implement Insertion sort algorithm in Java? (solution)
• How to find all pairs in an array of ints whose sum is equal to k (solution)
• How to remove duplicates from an array in Java? (solution)
• How to check if the linked list contains a loop or not? (algorithm)

If you like this tutorial and are interested in more Java tutorials on Data Structure and algorithmic concepts then Please share this with your friends and colleagues. That makes a difference and I really appreciate it.

VJosh said...

please put some more algorithm questions and how they can be implemented in java or else the explanation to various algorithms and why they are so popular?

Jane Rose said...

Thanks

javin paul said...

Unknown said...

Hi Javin,

Will the BST allows child value equal to it's parent?
I read somewhere that this is a special case and we need to handle this specially?

I'm confused as you said equal values of parent and child are allowed in BST

Thanks

javin paul said...

Hello kadam, if duplicate keys are allowed in binary search tree (BST), then nodes with values that are equal to the key in node n (parent node) can be either in n's left subtree or in its right subtree (but not both).

friday sze said...

i can get BST size right by using your getSize() method. It will be stuck in the lowest level of tree, which is 3, 1, 4, and the iteration will be infinite-loop .