BinaryTreesB.java

import java.util.*;
import java.util.LinkedList;
public class BinaryTreesB {
    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 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 void preorder(Node root) {
            if (root == null) {
                return;
            }
            System.out.print(root.data + " ");
            preorder(root.left);
            preorder(root.right);
        }

        public void inorder(Node root) {
            if (root == null) {
                return;
            }
            inorder(root.left);
            System.out.print(root.data + " ");
            inorder(root.right);
        }

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

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

        public 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);
    }
}