Flatten 2D Vector

601. Flatten 2D Vector

Implement an iterator to flatten a 2d vector.

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].

public class Vector2D implements Iterator<Integer> {
    private Iterator<List<Integer>> i;
    private Iterator<Integer> j;

    public Vector2D(List<List<Integer>> vec2d) {
        i = vec2d.iterator();
        j = null;
    }

    @Override
    public Integer next() {
        if (hasNext()) {
            return j.next();
        }
        
        return null;
    }

    @Override
    public boolean hasNext() {
        while ((j == null || !j.hasNext()) && i.hasNext()) {
            j = i.next().iterator();
        }
        
        return j != null && j.hasNext();
    }

    @Override
    public void remove() {
        if (j != null) {
            j.remove();
        }
    }
}

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

Hope this helps,
Michael