Recursive and iterative PostOrder tree traversal

PostOrder.java

//TODO add iterative with single stack
import java.util.*;
//Node class
class Node{
		 int data;
		 Node left;
		 Node right;
		Node(int data){
			this.data = data;
			this.left=null;
			this.right = null;
		}
	}
public class PostOrder {
	
	public static void main(String args[]){
		Node root = new Node(1);
		root.left = new Node(2);
		root.right = new Node(3);
		//root.left.left = new Node(4);
		root.left.right = new Node(5);
		root.right.left = new Node(6);
		root.right.right = new Node(7);
		iterativePostOrder(root);
		System.out.println();
		recursivePostOrder(root);
		
	}
//recursive method
	private static void recursivePostOrder(Node root) {
		// TODO Auto-generated method stub
		if(root != null){
			recursivePostOrder(root.left);
			recursivePostOrder(root.right);
			System.out.print(root.data+" ");

		}
		//System.out.println();
		
	}
//iterative method
	private static void iterativePostOrder(Node root) {
		// TODO Auto-generated method stub
		if(root == null)
			return;
		Stack<Node> s1 = new Stack<Node>();
		Stack<Node> s2 = new Stack<Node>();
		s1.push(root);
		while(!s1.isEmpty()){
			Node t = s1.pop();
			
			if(t.left!=null)s1.push(t.left);
			if(t.right!=null)s1.push(t.right);
			s2.push(t);
		}
		while(!s2.isEmpty()){
			System.out.print(s2.pop().data);
		}
		
	}
}