Zigzag Iterator

Given two 1d vectors, implement an iterator to return their elements alternately.

For example, given two 1d vectors:

v1 = [1, 2]
v2 = [3, 4, 5, 6]

By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1, 3, 2, 4, 5, 6].

Follow up: What if you are given k 1d vectors? How well can your code be extended to such cases?

Clarification for the follow up question - Update (2015-09-18):
The "Zigzag" order is not clearly defined and is ambiguous for k > 2 cases. If "Zigzag" does not look right to you, replace "Zigzag" with "Cyclic". For example, given the following input:

[1,2,3]
[4,5,6,7]
[8,9]

It should return [1,4,8,2,5,9,3,6,7].

public class ZigzagIterator {
    private int index1 = 0, index2 = 0;
    private List<Integer> l1;
    private List<Integer> l2;
    
    public ZigzagIterator(List<Integer> v1, List<Integer> v2) {
        l1 = v1;
        l2 = v2;
    }

    public int next() {
        if (index1 < l1.size()) {
            if (index2 < l2.size()) {
                if (index2 < index1) return l2.get(index2++);
            }
            return l1.get(index1++);
        } else return l2.get(index2++);
    }

    public boolean hasNext() {
        return index1 < l1.size() || index2 < l2.size();
    }
}

/**
 * Your ZigzagIterator object will be instantiated and called as such:
 * ZigzagIterator i = new ZigzagIterator(v1, v2);
 * while (i.hasNext()) v[f()] = i.next();
 */