This problem was asked by Uber.
Implement a 2D iterator class. It will be initialized with an array of arrays, and should implement the following methods:
next(): returns the next element in the array of arrays. If there are no more elements, raise an exception.has_next(): returns whether or not the iterator still has elements left.
For example, given the input [[1, 2], [3], [], [4, 5, 6]], calling next() repeatedly should output 1, 2, 3, 4, 5, 6.
Do not use flatten or otherwise clone the arrays. Some of the arrays can be empty.