Flatten 2D Vector

Description

Implement an iterator to flatten a 2d vector.

For example, Given 2d vector =

[ [1,2], [3], [4,5,6] ] By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,2,3,4,5,6].

Hint

How many variables do you need to keep track? Two variables is all you need. Try with x and y. Beware of empty rows. It could be the first few rows. To write correct code, think about the invariant to maintain. What is it?

Train of Thought

Put all iterator in a queue Keep track of the current iterator Check hasNext() and next() of current

Code

/**
 * Your Vector2D object will be instantiated and called as such:
 * Vector2D i = new Vector2D(vec2d);
 * while (i.hasNext()) v[f()] = i.next();
 */
public class Vector2D {

    Queue<Iterator<Integer>> queue;
    Iterator<Integer> current = null;

    public Vector2D(List<List<Integer>> vec2d) {
        queue = new LinkedList<Iterator<Integer>>();
        for (int i = 0; i < vec2d.size(); i++){
            queue.add(vec2d.get(i).iterator());
        }
        current = queue.poll(); // first
    }

    public int next() {
        if (!current.hasNext()) return -1;

        return current.next();
    }

    public boolean hasNext() {
        if (current == null) return false;

        while (!current.hasNext()) {
            if (!queue.isEmpty()) {
                current = queue.poll();
            } else return false;
        }

        return true;
    }
}

Complexity