Java program for iterative post order using two stacks

PostOrderIterative.java

// Java program for iterative post order using two stacks
// http://www.geeksforgeeks.org/iterative-postorder-traversal/
import java.util.*;
public class IterativePostorder 
{

	static class node 
	{
		int data;
		node left, right;

		public node(int data) 
		{
			this.data = data;
		}
	}

	// Two stacks as used in explanation
	static Stack<node> s1, s2;

	static void postOrderIterative(node root) 
	{
		// Create two stacks
		s1 = new Stack<>();
		s2 = new Stack<>();

		if (root == null)
			return;

		// push root to first stack
		s1.push(root);
		
		// Run while first stack is not empty
		while (!s1.isEmpty()) 
		{
			// Pop an item from s1 and push it to s2
			node temp = s1.pop();
			s2.push(temp);
		
			// Push left and right children of 
			// removed item to s1
			if (temp.left != null)
				s1.push(temp.left);
			if (temp.right != null)
				s1.push(temp.right);
		}

		// Print all elements of second stack
		while (!s2.isEmpty()) 
		{
			node temp = s2.pop();
			System.out.print(temp.data + " ");
		}
	}

	public static void main(String[] args) 
	{
		node root = null;
		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);

		postOrderIterative(root);
	}
}