Data structure: insert, remove, contains, get random element, all at O(1)

Question

I was given this problem in an interview. How would you have answered?

Design a data structure that offers the following operations in O(1) time:

• inse

Answer 1

I agree with Anon. Except for the last requirement where getting a random element with equal fairness is required all other requirements can be addressed only using a single Hash based DS. I will choose HashSet for this in Java. The modulo of hash code of an element will give me the index no of the underlying array in O(1) time. I can use that for add, remove and contains operations.