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public class Vector2D {
    int indexList, indexEle;
    List<List<Integer>> vec; 
    
    public Vector2D(List<List<Integer>> vec2d) {
        indexList = 0;
        indexEle = 0;
        vec = vec2d;
    }

    public int next() {
        return vec.get(indexList).get(indexEle++);
    }

    public boolean hasNext() {
        while(indexList < vec.size()){
            if(indexEle < vec.get(indexList).size())
                return true;
            else{
                indexList++;
                indexEle = 0;
            }
        }
        return false;
    }
}

public class Vector2D implements Iterator<Integer> {
    
    private Iterator<List<Integer>> row;
    private Iterator<Integer> col;
    public Vector2D(List<List<Integer>> vec2d) {
        row = vec2d.iterator();
        //col = row.hasNext() ? row.next() : null;
    }

    @Override
    public Integer next() {
        hasNext();
        return col.next();
    }

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

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