251. Flatten 2D Vector

class Vector2D {
    
    private List<Integer> flattenVec;
    private int index;

    public Vector2D(int[][] vec) {
        flattenVec = new ArrayList<>();
        for (int i = 0; i < vec.length; i++) {
            for (int j = 0; j < vec[i].length; j++) {
                flattenVec.add(vec[i][j]);
            }
        }
        index = 0;
    }
    
    public int next() {
        int val = flattenVec.get(index);
        index ++;
        return val;
    }
    
    public boolean hasNext() {
        return index < flattenVec.size();
    }
}

/**
 * Your Vector2D object will be instantiated and called as such:
 * Vector2D obj = new Vector2D(vec);
 * int param_1 = obj.next();
 * boolean param_2 = obj.hasNext();
 */