buildtree.java

import java.util.*;

public class buildtree {
  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){
    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+" ");
  System.out.println();
  
}

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};
    int k = 2;
     BinaryTree tree = new BinaryTree();
     Node root = tree.buildTree(nodes);
     levelOrder(root);
     
  }
}