In this post, we will write a Java program to implement a Binary tree.

 A binary tree is a recursive data structure where each node can have 2 children at most.

A common type of binary tree is a binary search tree, in which every node has a value that is greater than or equal to the node values in the left sub-tree, and less than or equal to the node values in the right sub-tree.

Binary Tree Program in Java

Here is the Binary tree program to implement a sorted binary tree in Java, and its most common operations.

package com.java.collections;

import java.util.LinkedList;
import java.util.Queue;
import java.util.Stack;

public class BinaryTree {

    Node root;

    public void add(int value) {
        root = addRecursive(root, value);
    }

    private Node addRecursive(Node current, int value) {

        if (current == null) {
            return new Node(value);
        }

        if (value < current.value) {
            current.left = addRecursive(current.left, value);
        } else if (value > current.value) {
            current.right = addRecursive(current.right, value);
        }

        return current;
    }

    public boolean isEmpty() {
        return root == null;
    }

    public int getSize() {
        return getSizeRecursive(root);
    }

    private int getSizeRecursive(Node current) {
        return current == null ? 0 : getSizeRecursive(current.left) + 1 + getSizeRecursive(current.right);
    }

    public boolean containsNode(int value) {
        return containsNodeRecursive(root, value);
    }

    private boolean containsNodeRecursive(Node current, int value) {
        if (current == null) {
            return false;
        }

        if (value == current.value) {
            return true;
        }

        return value < current.value
          ? containsNodeRecursive(current.left, value)
          : containsNodeRecursive(current.right, value);
    }

    public void delete(int value) {
        root = deleteRecursive(root, value);
    }

    private Node deleteRecursive(Node current, int value) {
        if (current == null) {
            return null;
        }

        if (value == current.value) {
            // Case 1: no children
            if (current.left == null && current.right == null) {
                return null;
            }

            // Case 2: only 1 child
            if (current.right == null) {
                return current.left;
            }

            if (current.left == null) {
                return current.right;
            }

            // Case 3: 2 children
            int smallestValue = findSmallestValue(current.right);
            current.value = smallestValue;
            current.right = deleteRecursive(current.right, smallestValue);
            return current;
        }
        if (value < current.value) {
            current.left = deleteRecursive(current.left, value);
            return current;
        }

        current.right = deleteRecursive(current.right, value);
        return current;
    }

    private int findSmallestValue(Node root) {
        return root.left == null ? root.value : findSmallestValue(root.left);
    }

    public void traverseInOrder(Node node) {
        if (node != null) {
            traverseInOrder(node.left);
            visit(node.value);
            traverseInOrder(node.right);
        }
    }

    public void traversePreOrder(Node node) {
        if (node != null) {
            visit(node.value);
            traversePreOrder(node.left);
            traversePreOrder(node.right);
        }
    }

    public void traversePostOrder(Node node) {
        if (node != null) {
            traversePostOrder(node.left);
            traversePostOrder(node.right);
            visit(node.value);
        }
    }

    public void traverseLevelOrder() {
        if (root == null) {
            return;
        }

        Queue<Node> nodes = new LinkedList<>();
        nodes.add(root);

        while (!nodes.isEmpty()) {

            Node node = nodes.remove();

            System.out.print(" " + node.value);

            if (node.left != null) {
                nodes.add(node.left);
            }

            if (node.left != null) {
                nodes.add(node.right);
            }
        }
    }

    
    public void traverseInOrderWithoutRecursion() {
        Stack<Node> stack = new Stack<Node>();
        Node current = root;
        stack.push(root);
        while(! stack.isEmpty()) {
            while(current.left != null) {
                current = current.left;                
                stack.push(current);                
            }
            current = stack.pop();
            visit(current.value);
            if(current.right != null) {
                current = current.right;                
                stack.push(current);
            }
        }
    }
    
    public void traversePreOrderWithoutRecursion() {
        Stack<Node> stack = new Stack<Node>();
        Node current = root;
        stack.push(root);
        while(! stack.isEmpty()) {
            current = stack.pop();
            visit(current.value);
            
            if(current.right != null)
                stack.push(current.right);
                
            if(current.left != null)
                stack.push(current.left);
        }        
    }
    
    public void traversePostOrderWithoutRecursion() {
        Stack<Node> stack = new Stack<Node>();
        Node prev = root;
        Node current = root;
        stack.push(root);

        while (!stack.isEmpty()) {
            current = stack.peek();
            boolean hasChild = (current.left != null || current.right != null);
            boolean isPrevLastChild = (prev == current.right || (prev == current.left && current.right == null));

            if (!hasChild || isPrevLastChild) {
                current = stack.pop();
                visit(current.value);
                prev = current;
            } else {
                if (current.right != null) {
                    stack.push(current.right);
                }
                if (current.left != null) {
                    stack.push(current.left);
                }
            }
        }   
    }    
    
    private void visit(int value) {
        System.out.print(" " + value);        
    }
    
    class Node {
        int value;
        Node left;
        Node right;

        Node(int value) {
            this.value = value;
            right = null;
            left = null;
        }
    }
    
    public static void main(String arg[]){
         BinaryTree bt = new BinaryTree();

        bt.add(6);
        bt.add(4);
        bt.add(8);
        bt.add(3);
        bt.add(5);
        bt.add(7);
        bt.add(9);
        
        bt.traversePostOrder(bt.root);
        System.out.println();
        bt.traversePostOrderWithoutRecursion();
        
        bt.traverseInOrder(bt.root);
        System.out.println();
        bt.traverseInOrderWithoutRecursion();
        
        bt.traversePreOrder(bt.root);
        System.out.println();
        bt.traversePreOrderWithoutRecursion();
        
        bt.traverseLevelOrder();
        
    }
}

Output:

 3 5 4 7 9 8 6
 3 5 4 7 9 8 6 3 4 5 6 7 8 9
 3 4 5 6 7 8 9 6 4 3 5 8 7 9
 6 4 3 5 8 7 9 6 4 8 3 5 7 9