a_152_BinaryTreeB.java

import java.util.*;

public class a_152_BinaryTreeB {

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

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

    }

    static class BinaryTree {
        static int idx = -1 ;
        public static Node buildTree(int nodes[]) {
            idx++ ;
            if(nodes[idx] == -1 ) {
                return null ;
            }

            Node newNode = new Node(nodes[idx] ) ;
            newNode.left = buildTree(nodes) ;
            newNode.right = buildTree(nodes) ;

            return newNode ;
        }

        public static void preorder(Node root) {
            if(root == null){
                System.out.print("-1 ");
                return ;
            }

            System.out.print(root.data + " ");
            preorder(root.left);
            preorder(root.right);
        }

        public static void inorder(Node root){
            if(root == null){
                return ;
            }

            inorder(root.left);
            System.out.print(root.data +" ");
            inorder(root.right);
        }

        public static void postorder(Node root){
            if(root == null){
                return ;
            }

            postorder(root.left);
            postorder(root.right);
            System.out.print(root.data +" ");
        }
        
        // Level Order Traversal 
        public static void levelOrder(Node root) {
            if(root == null ) {
                return ;
            }

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

            while(!q.isEmpty()) {
                Node currNode = q.remove() ;

                if(currNode == null ){
                    System.out.println();
                    if(q.isEmpty()) {
                        break ;
                    }
                    else{
                        q.add(null) ;
                    }
                }
                else{
                    System.out.print(currNode.data +" ");
                    if(currNode.left != null) {
                        q.add(currNode.left) ;
                    }
                    if(currNode.right != null) {
                        q.add(currNode.right ) ;
                    }
                }
            }


        }



    }


    public static void main(String[] args) {
        int nodes[] = { 1, 2, 4, -1, -1, 5, -1, -1, 3, -1, 6, -1, -1 } ;
        BinaryTree tree = new BinaryTree() ;
        Node root = tree.buildTree(nodes)  ;
    
        tree.levelOrder(root);

    }
}