Implement an iterator to flatten a 2d vector.

For example,
Given 2d vector =

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[
  [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].

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public class Vector2D implements Iterator<Integer> {
    Iterator<List<Integer>> iter;
    Iterator<Integer> cur;

    public Vector2D(List<List<Integer>> vec2d) {
        iter = vec2d.iterator();
        cur = iter.hasNext() ? iter.next().iterator() : null;
    }

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

    @Override
    public boolean hasNext() {
        // using while is neccesary for case like [[], [], [-1]]
        // note double paranthesis below
        while ((cur == null || !cur.hasNext()) && iter.hasNext()){
            cur = iter.next().iterator();
        }
        return cur != null && cur.hasNext();
    }
}

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