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

/**
 * An iterator for binary trees that visits elements in a
 * pre-order manner (current node, then left subtree, then right subtree).
 * Basic idea is to maintain a stack of nodes to visit, and each time
 * a tree/subtree is processed, first visit the node itself, then add the right
 * and left subtrees to the todo list (stack).
 */
public class PreorderIterator<T> implements Iterator<T> {
    
    // stack of nodes to visit
    private Deque<BinaryTree<T>> todo;

    // set up the stack
    public PreorderIterator(BinaryTree<T> tree) {
        todo = new ArrayDeque<BinaryTree<T>>();
        todo.push(tree);
    }

    // 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();
        
        // push right subtree
        if (!top.right().isEmpty()) {
            todo.push(top.right());
        }
        
        // push left subtree (LIFO ordering, so this is next)
        if (!top.left().isEmpty()) {
            todo.push(top.left());
        }
        
        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);
        
        PreorderIterator<String> iter = new PreorderIterator(green);
        while (iter.hasNext()) {
            System.out.println(iter.next());
        }
        
    }

}