Flatten 2D Vector

You are given a 2D vector `vec`, represented as a list of lists of integers. Design an iterator that flattens the 2D vector and iterates through all the elements in row-major order (i.e., from left to right and top to bottom). Implement the `Vector2D` class: - `Vector2D(int[][] vec)` initializes the object with the 2D vector `vec`. - `int next()` returns the next element in the flattened vector. - `boolean hasNext()` returns _true_ if there are still elements to be returned, or _false_ otherwise. Your implementation should support calling `next()` and `hasNext()` multiple times and in any order. Return the appropriate values for each operation as described above.