#Problem Statement
To check whether a one time coupon has already been used. If so, a suitable message be presented to user. Otherwise the system, should inform the user that the coupan is valid and it should be marked as used and cannot be used again. Due to high number of coupons, it is necessary to choose a data structure that allows for quickly checking whether an element exists in some collection. For this reason, that hash set is chosen as a data structure for storing identifiers of the used coupons. Therfore, you just need to check whether an entered identifiers exists in the set.