LeetCode-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].

Follow up:
As an added challenge, try to code it using only iterators in C++ or iterators in Java.

Solution:

public class Vector2D implements Iterator<Integer> {
    List<List<Integer>> vecs;
    int curIndex;
    Iterator<Integer> iter;

    public Vector2D(List<List<Integer>> vec2d) {
        vecs = vec2d;
        curIndex = 0;
        if (curIndex >= vecs.size()) return;
        iter = vecs.get(curIndex).iterator();
        moveToNext();
    }

    private void moveToNext() {
        while (!iter.hasNext()) {
            curIndex++;
            if (curIndex >= vecs.size())
                break;
            iter = vecs.get(curIndex).iterator();
        }
    }

    @Override
    public Integer next() {
        if (curIndex >= vecs.size())
            return null;

        Integer res = iter.next();
        moveToNext();
        
        return res;
    }

    @Override
    public boolean hasNext() {
        return curIndex < vecs.size();
    }
}