281. Zigzag Iterator [https://leetcode.com/problems/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