Design a Data Structure with Insert, Remove, and GetRandom for Unique Elements in O(1)

Design a data structure that supports three operations, all in average O(1) time: - insert(val): adds val to the data structure if not already present - remove(val): removes val if present - getUnique(): returns a uniformly random element that has been inserted exactly once (i.e., never re-inserted after removal) For example, after insert(1), insert(2), insert(1): getUnique() must return 2, since 1 was inserted more than once. After remove(1), both 1 and 2 are unique again, so getUnique() may return either.