Zigzag Iterator

Given two 1d vectors, implement an iterator to return their elements alternately.
For example, given two 1d vectors:
1. v1 = [1, 2]
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?
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. [1,2,3]
2. [4,5,6,7]
3. [8,9]

It should return [1,4,8,2,5,9,3,6,7].
Solution:
1. public class ZigzagIterator {
2. int i1 = 0;
3. int i2 = 0;
4. 5. boolean flag = false;
6. 7. List l1;
8. List l2;
9. 10. public ZigzagIterator(List v1, List v2) {
11. l1 = v1;
12. l2 = v2;
13. }
14. 15. public int next() {
16. flag = !flag;
17. 18. if (i1 < l1.size() && (flag || i2 >= l2.size()))
19. return l1.get(i1++);
20. 21. if (i2 < l2.size() && (!flag || i1 >= l1.size()))
22. return l2.get(i2++);
23. 24. return -1;
25. }
26. 27. public boolean hasNext() {
28. return i1 < l1.size() || i2 < l2.size();
29. }
30. }
31. 32. /*
33. * Your ZigzagIterator object will be instantiated and called as such:
34. * ZigzagIterator i = new ZigzagIterator(v1, v2);
35. * while (i.hasNext()) v[f()] = i.next();
36. /