import java.util.Iterator;
import java.util.Deque;
import java.util.ArrayDeque;

/**
 * An iterator for binary trees that visits elements in an
 * in-order manner (current node, then left subtree, then right subtree).
 * Here, the key idea is that when we process a tree/subtree, *before*
 * we visit the node itself, we first walk through all left descendants
 * adding them to the todo list.
 */
public class InorderIterator<T> implements Iterator<T> {

    // stack of nodes to visit
    private Deque<BinaryTree<T>> todo;

    // set up the stack
    public InorderIterator(BinaryTree<T> tree) {
        todo = new ArrayDeque<BinaryTree<T>>();

        // push all left descendants into todo
        processLeft(tree);
    }

    // push all left descendants starting at current onto todo
    private void processLeft(BinaryTree<T> current) {
        while (!current.isEmpty()) {
            todo.push(current);
            current = current.left();
        }
    }

    // not done while the stack still has nodes to visit
    public boolean hasNext() {
        return !todo.isEmpty();
    }

    public T next() {
        // top value is the next element
        BinaryTree<T> top = todo.pop();
        T result = top.value();

        // the left subtree was already processed previously,
        // so we only need to worry about the right subtree now
        if (!top.right().isEmpty()) {
            // since this is an in-order traversal,
            // we push all left descendants (just as we did with the root).
            processLeft(top.right());
        }

        return result;
    }

    public static void main(String args[]) {
        BinaryTree<String> blue = new BinaryTree<String>("blue");
        BinaryTree<String> indigo = new BinaryTree<String>("indigo");
        BinaryTree<String> red = new BinaryTree<String>("red");
        BinaryTree<String> yellow = new BinaryTree<String>("yellow");

        BinaryTree<String> orange = new BinaryTree<String>("orange", indigo, red);
        BinaryTree<String> violet = new BinaryTree<String>("violet", orange, yellow);
        BinaryTree<String> green = new BinaryTree<String>("green", blue, violet);

        InorderIterator<String> iter = new InorderIterator(green);
        while (iter.hasNext()) {
            System.out.println(iter.next());
        }

    }

}