//Given a nested list of integers, implement an iterator to flatten it.
//
//Each element is either an integer, or a list -- whose elements may also be integers or other lists.
//
//Example 1:
//Given the list [[1,1],2,[1,1]],
//
//By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].