Earliest User Who Logged In Exactly Once

Problem: Earliest User Who Logged In Exactly Once (OOD / Data-Structure Design) You are given a stream/list of user login events. Each event contains: user_id: string or integer login_time: integer timestamp (e.g., seconds) Design a data structure/class that supports: add_login(user_id, login_time): add a login event query(): return the user whose total login count is exactly 1 and whose login_time is the earliest among such users. If no such user exists, return null (or empty / -1, define your convention). Requirements Provide and implement the core data structure design. Analyze time and space complexity. Constraints (can be clarified during interview) Number of events N up to around 1e5 Number of distinct users up to around 1e5 login_time is a non-negative integer Example Tests Input: add: (A, 10), (B, 5), (A, 11), (C, 7) query() Output: B Input: add: (A, 10), (A, 9) query() Output: null Input: add: (A, 10), (B, 10) query() Output: A or B (depending on your tie-break rule) Example Input add A 10 add B 5 add A 11 add C 7 query Output B