How to implement a reverse iterator (PostOrder) to traverse a binary tree in Java?

Question

Java

Tell me how to implement a reverse iterator (PostOrder) to traverse a binary tree in Java, without using stack and other collections.
There is a class:

class Node{
    Node left;
    Node right;
    Node parent;
    int val;
}

And interface:

interface CIterator{
    bool hasNext();
    int next();
}

And you need to implement the class:

class PostOrderIterator implements CIterator{
    Node n;
    public PostOrderIterator(Node k){
        n = k;
    }
}

Answer 1

What type of tree traversal do you generally use?
And why create your own interfaces when there is a ListIterator?