I need some help for public class PostorderIterator implements Iterator

public class BinaryTree {

//Implements a Binary Tree of Strings

private class Node {

private Node left;

private String data;

private Node right;

private Node parent; // reference to the parent node

// the parent is null for the root node

private Node(Node L, String d, Node r, Node p) {

left = L;

data = d;

right = r;

parent = p;

}

}

private Node root;

public BinaryTree() {

// create an empty tree

root = null;

}

public BinaryTree(String d) {

// create a tree with a single node

root = new Node(null, d, null, root);

}

public class PostorderIterator implements Iterator {

//An iterator that returns data in the tree in a post order pattern

//This implementation must use a stack and must not use the parent pointer

//You must use Javas stack class

public PostorderIterator() {

}

public boolean hasNext() {

}

public String next() {

}

public void remove() {

//optional method not implemented

}

}

public Iterator postorder() {

// return a new post order iterator object

}