Flatten 2D Vector

public class Vector2D {
    Iterator<Integer> iterator;
    Queue<iterator> q = new LinkedList<iterator>();//用queue比用ArrayList方便
    public Vector2D(List<List<Integer>> vec2d) {
        for (List<Integer> vec : vec2d) {
            q.offer(vec.iterator());
        }
        if (!q.isEmpty()) {
            iterator = q.poll();
        }
    }

    public int next() {
        if (hasNext()) {
            return iterator.next();
        } else {
            return -1;
        }
    }

    public boolean hasNext() {
        if (iterator == null) return false;
        while (!iterator.hasNext() && !q.isEmpty()) {
            iterator = q.poll();
        }
        return iterator.hasNext();
    }
}