**This Tutorial Covers Binary Search Tree in Java. You will learn to Create a BST, Insert, Remove and Search an Element, Traverse & Implement a BST in Java:**

A Binary search tree (referred to as BST hereafter) is a type of binary tree. It can also be defined as a node-based binary tree. BST is also referred to as ‘Ordered Binary Tree’. In BST, all the nodes in the left subtree have values that are less than the value of the root node.

Similarly, all the nodes of the right subtree of the BST have values that are greater than the value of the root node. This ordering of nodes has to be true for respective subtrees as well.

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What You Will Learn:

## Binary Search Tree In Java

A BST does not allow duplicate nodes.

**The below diagram shows a BST Representation:**

Above shown is a sample BST. We see that 20 is the root node of this tree. The left subtree has all the node values that are less than 20. The right subtree has all the nodes that are greater than 20. We can say that the above tree fulfills the BST properties.

The BST data structure is considered to be very efficient when compared to Arrays and Linked list when it comes to insertion/deletion and searching of items.

BST takes O (log n) time to search for an element. As elements are ordered, half the subtree is discarded at every step while searching for an element. This becomes possible because we can easily determine the rough location of the element to be searched.

Similarly, insertion and deletion operations are more efficient in BST. When we want to insert a new element, we roughly know in which subtree (left or right) we will insert the element.

### Creating A Binary Search Tree (BST)

Given an array of elements, we need to construct a BST.

**Let's do this as shown below:**

**Given array:** 45, 10, 7, 90, 12, 50, 13, 39, 57

Let's first consider the top element i.e. 45 as the root node. From here we will go on creating the BST by considering the properties already discussed.

To create a tree, we will compare each element in the array with the root. Then we will place the element at an appropriate position in the tree.

**The entire creation process for BST is shown below.**

**Binary Search Tree Operations**

BST supports various operations. The following table shows the methods supported by BST in Java. We will discuss each of these methods separately.

Method/operation | Description |
---|---|

Insert | Add an element to the BST by not violating the BST properties. |

Delete | Remove a given node from the BST. The node can be the root node, non-leaf, or leaf node. |

Search | Search the location of the given element in the BST. This operation checks if the tree contains the specified key. |

#### Insert An Element In BST

An element is always inserted as a leaf node in BST.

**Given below are the steps for inserting an element.**

- Start from the root.
- Compare the element to be inserted with the root node. If it is less than root, then traverse the left subtree or traverse the right subtree.
- Traverse the subtree till the end of the desired subtree. Insert the node in the appropriate subtree as a leaf node.

**Let's see an illustration of the insert operation of BST.**

Consider the following BST and let us insert element 2 in the tree.

The insert operation for BST is shown above. In fig (1), we show the path that we traverse to insert element 2 in the BST. We have also shown the conditions that are checked at each node. As a result of the recursive comparison, element 2 is inserted as the right child of 1 as shown in fig (2).

#### Search Operation In BST

To search if an element is present in the BST, we again start from the root and then traverse the left or right subtree depending on whether the element to be searched is less than or greater than the root.

**Enlisted below are the steps that we have to follow.**

- Compare the element to be searched with the root node.
- If the key (element to be searched) = root, return root node.
- Else if key < root, traverse the left subtree.
- Else traverse right subtree.
- Repetitively compare subtree elements until the key is found or the end of the tree is reached.

Let's illustrate the search operation with an example. Consider that we have to search the key = 12.

**In the below figure, we will trace the path we follow to search for this element.**

As shown in the above figure, we first compare the key with root. Since the key is greater, we traverse the right subtree. In the right subtree, we again compare the key with the first node in the right subtree.

We find that the key is less than 15. So we move to the left subtree of node 15. The immediate left node of 15 is 12 that matches the key. At this point, we stop the search and return the result.

#### Remove Element From The BST

When we delete a node from the BST, then there are three possibilities as discussed below:

**Node Is A Leaf Node**

If a node to be deleted is a leaf node, then we can directly delete this node as it has no child nodes. This is shown in the below image.

As shown above, the node 12 is a leaf node and can be deleted straight away.

**Node Has Only One Child**

When we need to delete the node that has one child, then we copy the value of the child in the node and then delete the child.

In the above diagram, we want to delete node 90 which has one child 50. So we swap the value 50 with 90 and then delete node 90 which is a child node now.

**Node Has Two Children**

When a node to be deleted has two children, then we replace the node with the inorder (left-root-right) successor of the node or simply said the minimum node in the right subtree if the right subtree of the node is not empty. We replace the node with this minimum node and delete the node.

In the above diagram, we want to delete node 45 which is the root node of BST. We find that the right subtree of this node is not empty. Then we traverse the right subtree and find that node 50 is the minimum node here. So we replace this value in place of 45 and then delete 45.

If we check the tree, we see that it fulfills the properties of a BST. Thus the node replacement was correct.

### Binary Search Tree (BST) Implementation In Java

The following program in Java provides a demonstration of all the above BST operation using the same tree used in illustration as an example.

class BST_class { //node class that defines BST node class Node { int key; Node left, right; public Node(int data){ key = data; left = right = null; } } // BST root node Node root; // Constructor for BST =>initial empty tree BST_class(){ root = null; } //delete a node from BST void deleteKey(int key) { root = delete_Recursive(root, key); } //recursive delete function Node delete_Recursive(Node root, int key) { //tree is empty if (root == null) return root; //traverse the tree if (key < root.key) //traverse left subtree root.left = delete_Recursive(root.left, key); else if (key > root.key) //traverse right subtree root.right = delete_Recursive(root.right, key); else { // node contains only one child if (root.left == null) return root.right; else if (root.right == null) return root.left; // node has two children; //get inorder successor (min value in the right subtree) root.key = minValue(root.right); // Delete the inorder successor root.right = delete_Recursive(root.right, root.key); } return root; } int minValue(Node root) { //initially minval = root int minval = root.key; //find minval while (root.left != null) { minval = root.left.key; root = root.left; } return minval; } // insert a node in BST void insert(int key) { root = insert_Recursive(root, key); } //recursive insert function Node insert_Recursive(Node root, int key) { //tree is empty if (root == null) { root = new Node(key); return root; } //traverse the tree if (key < root.key) //insert in the left subtree root.left = insert_Recursive(root.left, key); else if (key > root.key) //insert in the right subtree root.right = insert_Recursive(root.right, key); // return pointer return root; } // method for inorder traversal of BST void inorder() { inorder_Recursive(root); } // recursively traverse the BST void inorder_Recursive(Node root) { if (root != null) { inorder_Recursive(root.left); System.out.print(root.key + " "); inorder_Recursive(root.right); } } boolean search(int key) { root = search_Recursive(root, key); if (root!= null) return true; else return false; } //recursive insert function Node search_Recursive(Node root, int key) { // Base Cases: root is null or key is present at root if (root==null || root.key==key) return root; // val is greater than root's key if (root.key > key) return search_Recursive(root.left, key); // val is less than root's key return search_Recursive(root.right, key); } } class Main{ public static void main(String[] args) { //create a BST object BST_class bst = new BST_class(); /* BST tree example 45 / \ 10 90 / \ / 7 12 50 */ //insert data into BST bst.insert(45); bst.insert(10); bst.insert(7); bst.insert(12); bst.insert(90); bst.insert(50); //print the BST System.out.println("The BST Created with input data(Left-root-right):"); bst.inorder(); //delete leaf node System.out.println("\nThe BST after Delete 12(leaf node):"); bst.deleteKey(12); bst.inorder(); //delete the node with one child System.out.println("\nThe BST after Delete 90 (node with 1 child):"); bst.deleteKey(90); bst.inorder(); //delete node with two children System.out.println("\nThe BST after Delete 45 (Node with two children):"); bst.deleteKey(45); bst.inorder(); //search a key in the BST boolean ret_val = bst.search (50); System.out.println("\nKey 50 found in BST:" + ret_val ); ret_val = bst.search (12); System.out.println("\nKey 12 found in BST:" + ret_val ); } }

**Output: **

### Binary Search Tree (BST) Traversal In Java

A tree is a hierarchical structure, thus we cannot traverse it linearly like other data structures such as arrays. Any type of tree needs to be traversed in a special way so that all its subtrees and nodes are visited at least once.

Depending on the order in which the root node, left subtree and right subtree are traversed in a tree, there are certain traversals as shown below:

- Inorder Traversal
- Preorder Traversal
- PostOrder Traversal

All the above traversals use depth-first technique i.e. the tree is traversed depthwise.

The trees also use the breadth-first technique for traversal. The approach using this technique is called **“Level Order”** traversal.

**In this section, we will demonstrate each of the traversals using following BST as an example.**

**With the BST as shown in the above diagram, the level order traversal for the above tree is :**

Level Order Traversal: 10 6 12 4 8

#### Inorder Traversal

The inorder traversal approach traversed the BST in the order, **Left subtree=>RootNode=>Right subtree**. The inorder traversal provides a decreasing sequence of nodes of a BST.

**The algorithm InOrder (bstTree) for InOrder Traversal is given below.**

- Traverse the left subtree using InOrder (left_subtree)
- Visit the root node.
- Traverse the right subtree using InOrder (right_subtree)

**The inorder traversal of the above tree is:**

4 6 8 10 12

As seen, the sequence of the nodes as a result of the inorder traversal is in decreasing order.

#### Preorder Traversal

In preorder traversal, the root is visited first followed by the left subtree and right subtree. Preorder traversal creates a copy of the tree. It can also be used in expression trees to obtain prefix expression.

**The algorithm for PreOrder (bst_tree) traversal is given below:**

- Visit the root node
- Traverse the left subtree with PreOrder (left_subtree).
- Traverse the right subtree with PreOrder (right_subtree).

**The preorder traversal for the BST given above is:**

10 6 4 8 12

#### PostOrder Traversal

The postOrder traversal traverses the BST in the order: **Left subtree->Right subtree->Root node**. PostOrder traversal is used to delete the tree or obtain the postfix expression in case of expression trees.

**The algorithm for postOrder (bst_tree) traversal is as follows:**

- Traverse the left subtree with postOrder (left_subtree).
- Traverse the right subtree with postOrder (right_subtree).
- Visit the root node

**The postOrder traversal for the above example BST is:**

4 8 6 12 10

Next, we will implement these traversals using the depth-first technique in a Java implementation.

//define node of the BST class Node { int key; Node left, right; public Node(int data){ key = data; left = right = null; } } //BST class class BST_class { // BST root node Node root; BST_class(){ root = null; } //PostOrder Traversal - Left:Right:rootNode (LRn) void postOrder(Node node) { if (node == null) return; // first traverse left subtree recursively postOrder(node.left); // then traverse right subtree recursively postOrder(node.right); // now process root node System.out.print(node.key + " "); } // InOrder Traversal - Left:rootNode:Right (LnR) void inOrder(Node node) { if (node == null) return; //first traverse left subtree recursively inOrder(node.left); //then go for root node System.out.print(node.key + " "); //next traverse right subtree recursively inOrder(node.right); } //PreOrder Traversal - rootNode:Left:Right (nLR) void preOrder(Node node) { if (node == null) return; //first print root node first System.out.print(node.key + " "); // then traverse left subtree recursively preOrder(node.left); // next traverse right subtree recursively preOrder(node.right); } // Wrappers for recursive functions void postOrder_traversal() { postOrder(root); } void inOrder_traversal() { inOrder(root); } void preOrder_traversal() { preOrder(root); } } class Main{ public static void main(String[] args) { //construct a BST BST_class tree = new BST_class(); /* 45 // \\ 10 90 // \\ 7 12 */ tree.root = new Node(45); tree.root.left = new Node(10); tree.root.right = new Node(90); tree.root.left.left = new Node(7); tree.root.left.right = new Node(12); //PreOrder Traversal System.out.println("BST => PreOrder Traversal:"); tree.preOrder_traversal(); //InOrder Traversal System.out.println("\nBST => InOrder Traversal:"); tree.inOrder_traversal(); //PostOrder Traversal System.out.println("\nBST => PostOrder Traversal:"); tree.postOrder_traversal(); } }

**Output:**

### Frequently Asked Questions

**Q #1) Why do we need a Binary Search Tree?**

**Answer**: The way we search for elements in the linear data structure like arrays using binary search technique, the tree being a hierarchical structure, we need a structure that can be used for locating elements in a tree.

This is where the Binary search tree comes that helps us in the efficient searching of elements into the picture.

**Q #2) What are the properties of a Binary Search Tree?**

**Answer**: **A Binary Search Tree that belongs to the binary tree category has the following properties:**

- The data stored in a binary search tree is unique. It doesn’t allow duplicate values.
- The nodes of the left subtree are less than the root node.
- The nodes of the right subtree are greater than the root node.

**Q #3) What are the applications of a Binary Search Tree?**

**Answer**: We can use Binary Search Trees to solve some continuous functions in mathematics. Searching of data in hierarchical structures becomes more efficient with Binary Search Trees. With every step, we reduce the search by half subtree.

**Q #4) What is the difference between a Binary Tree and a Binary Search Tree?**

**Answer:** A binary tree is a hierarchical tree structure in which each node known as the parent can at most have two children. A binary search tree fulfills all the properties of the binary tree and also has its unique properties.

In a binary search tree, the left subtrees contain nodes that are less than or equal to the root node and the right subtree has nodes that are greater than the root node.

**Q #5) Is Binary Search Tree Unique?**

**Answer**: A binary search tree belongs to a binary tree category. It is unique in the sense it doesn’t allow duplicate values and also all its elements are ordered according to specific ordering.

## Conclusion

Binary Search trees are a part of the binary tree category and are mainly used for searching hierarchical data. It is also used for solving some mathematical problems.

In this tutorial, we have seen the implementation of a Binary Search Tree. We have also seen various operations performed on BST with their illustration and also explored the traversals for BST.

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