# Java - How to Create a Binary Search Tree

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Join For FreeThis article represents the high level concept and code samples which could be used to create a binary search tree in Java. Please feel free to comment/suggest if I missed to mention one or more important points. Also, sorry for the typos.

Following are the key points described later in this article:

- What is a binary search tree?
- What are different kind of traversals?
- Code Samples

###### What is a binary search tree?

A binary search tree is a binary tree in which every node contains a key that satisfies following criteria:

- The key in left child is less than the key in the parent node
- The key in the right child is more than the parent node
- The left and right child are again binary search trees.

Following diagram represents a binary search tree:

###### What are different kind of traversals?

Following are three different kind of traversals:

**Preorder traversal**: In preorder traversal, the node is visted first and then, left and right sub-trees.**Inorder traversal**: In inorder traversal, the node is visited between left and right sub-tree.**Postorder traversal**: In postorder traversal, the node is visited after left and right subtrees.

###### Code Sample – How to Create a Binary Search Tree

If the numbers such as {20, 15, 200, 25, -5, 0, 100, 20, 12, 126, 1000, -150} are to be stored in a BinaryTree (represented by code below), following would get printed using different kind of traversal mechanism:

//Preorder traversal 20, 15, -5, -150, 0, 12, 200, 25, 20, 100, 126, 1000 // Inorder traversal -150, -5, 0, 12, 15, 20, 20, 25, 100, 126, 200, 1000 //Postorder traversal -150, 12, 0, -5, 15, 20, 126, 100, 25, 1000, 200, 20

Following is the code for creating binary tree that uses following BinaryTree class and traversals:

BinaryTree tree = new BinaryTree( 20 ); int[] nums = {15, 200, 25, -5, 0, 100, 20, 12, 126, 1000, -150}; for(int i : nums ) { tree.addNode( i ); } tree.traversePreOrder(); tree.traverseInOrder(); tree.traversePostOrder();

Following is the code for BinaryTree class:

public class BinaryTree { private int data; private BinaryTree left; private BinaryTree right; public BinaryTree(int num) { this.data = num; this.left = null; this.right = null; } // As a convention, if the key to be inserted is less than the key of root node, then key is inserted in // left sub-tree; If key is greater, it is inserted in right sub-tree. If it is equal, as a convention, it // is inserted in right sub-tree public void addNode(int num) { if (num < this.data) { if (this.left != null) { this.left.addNode(num); } else { this.left = new BinaryTree(num); } } else { if (this.right != null) { this.right.addNode(num); } else { this.right = new BinaryTree(num); } } } // Visit the node first, then left and right sub-trees public void traversePreOrder() { System.out.println( this.data ); if( this.left != null ) { this.left.traversePreOrder(); } if( this.right != null ) { this.right.traversePreOrder(); } } // Visit left sub-tree, then node and then, right sub-tree public void traverseInOrder() { if( this.left != null ) { this.left.traverseInOrder(); } System.out.println( this.data ); if( this.right != null ) { this.right.traverseInOrder(); } } // Visit left sub-tree, then right sub-tree and then the node public void traversePostOrder() { if( this.left != null ) { this.left.traversePostOrder(); } if( this.right != null ) { this.right.traversePostOrder(); } System.out.println( this.data ); } }

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