TreeBasic.java

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

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

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


public class TreeBasic {

    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) {  //Time complexity of preorder is O(N)

            if(root == null) {
                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+" ");
        }

        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);
        LevelOrder(root);
        
    }
    
}