You're building a registration system. When users register, if their username hasn't been taken, output OK. If it has, output the username with a number suffix. The first duplicate gets suffix 1, second gets 2, and so on.
This problem teaches you when maps beat sets. You need more than just presence checking. You need to track how many times each username appeared. Maps store key-value pairs, perfect for counting.
Before reading on, think: if three users try to register as "john", what should the outputs be? First gets OK, second gets john1, third gets john2. How would you track this state?