251. Flatten 2D Vector

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].

Attention:

1. filter out empty list when saving the incoming data;
2. in hasNext() function, make sure both pointers are valid;
3. after retrieve data in next(), update pointers.

public class Vector2D implements Iterator<Integer> {
    List<List<Integer>> vector = new ArrayList<>();
    int p =0;// which list;
    int q =0;// which number in list;
    public Vector2D(List<List<Integer>> vec2d) {
        for(List<Integer> l : vec2d){
            if(l.size()>0){
                vector.add(l);
            }
        }
    }

    @Override
    public Integer next() {
        int val = vector.get(p).get(q);
        q++;
        if(q == vector.get(p).size()){
            p++;
            q=0;
        }
        return val;
    }

    @Override
    public boolean hasNext() {

        if(p >= vector.size()) return false;
        if(p == (vector.size()-1) && q >= vector.get(p).size()) return false;
        else return true;
    }
}

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