Print Nodes Of Last Level Of BinaryTree

package tree;

public class Node{
 
 private Node left;
 private Node right;
 private int data;
 
 public Node(int data){
  this.data=data;
 }

 public Node getLeft() {
  return left;
 }

 public void setLeft(Node left) {
  this.left = left;
 }

 public Node getRight() {
  return right;
 }

 public void setRight(Node right) {
  this.right = right;
 }

 public int getData() {
  return data;
 }

 public void setData(int data) {
  this.data = data;
 }
}

package tree;

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

public class PrintNodesOfLastLevelOfBinaryTree {
 private Node rootNode;

 public static void main(String[] args) {
  new PrintNodesOfLastLevelOfBinaryTree();
 }

 public PrintNodesOfLastLevelOfBinaryTree(){

  addNode(rootNode, 50); 
  addNode(rootNode, 40); 
  addNode(rootNode, 60); 
  addNode(rootNode, 30); 
  addNode(rootNode, 45); 
  addNode(rootNode, 55); 
  addNode(rootNode, 70); 
  addNode(rootNode, 52); 
  addNode(rootNode, 58); 

  printTreeInOrder(rootNode);
  System.out.println();
  calculateAndPrintNodesAtEachLevelAnotherWay(rootNode);
 }

 private String calculateAndPrintNodesAtEachLevelAnotherWay(Node rootNode) {
  if(rootNode==null)
   return "";
  Queue<Node> q1 = new LinkedList<Node>();
  Queue<Node> q2 = new LinkedList<Node>();
  q1.add(rootNode);

  String str="";
  while(!q1.isEmpty()){

   Node temp = q1.poll();
   str+=temp.getData()+",";
   if(temp.getLeft()!=null){
    q2.add(temp.getLeft());
   }
   if(temp.getRight()!=null){
    q2.add(temp.getRight()); 
   }
   if(q1.isEmpty()){
    if(!q2.isEmpty()){
     //System.out.println(str);
     str="";
     q1=q2;
     q2=new LinkedList<Node>(); 
    }
   }
  }
  System.out.println(str);
  return null;
 }

 private void printTreeInOrder(Node rootNode){
  if(rootNode==null)
   return;
  printTreeInOrder(rootNode.getLeft());
  System.out.print(rootNode.getData() + " ");
  printTreeInOrder(rootNode.getRight());
 }

 private void addNode(Node rootNode, int data){
  if(rootNode==null){
   Node temp1 = new Node(data);
   this.rootNode=temp1;
  }else{
   addNodeInProperPlace(rootNode, data);
  }
 }

 private void addNodeInProperPlace(Node rootNode, int data){ 
  if(data>rootNode.getData()){
   if(rootNode.getRight()!=null){
    addNode(rootNode.getRight(), data);
   }else{
    Node temp1 = new Node(data);
    rootNode.setRight(temp1);
   }
  }else if(data<rootNode.getData()){
   if(rootNode.getLeft()!=null){
    addNode(rootNode.getLeft(), data);
   }else{
    Node temp1 = new Node(data);
    rootNode.setLeft(temp1);
   }
  }
 }
}