Email Subscription

Implement an email subscription management system. Users can subscribe to different topics and set notification schedules. When a subscription is about to expire, the system should send reminders based on the user's notification schedule. Specifications: Design the function subscribe(user_id, topic_id, expiry_date, schedule), where schedule is a string representing the reminder time, such as "1 day before". Design the function send_schedule() to check all users' subscriptions and send reminders. Consider the scenario where a user might have multiple subscriptions close to expiry, and handle this in send_schedule(). Input Format: subscribe: user ID, topic ID, expiration date, notification schedule. send_schedule: no input. Output Format: subscribe: no return. send_schedule: outputs all sent notification details. Constraints: Each user can subscribe to a maximum of 100 topics. Time complexity should be based on O(n) for scalability. Example Input subscribe('user1', 'topic1', '2025-12-10', '1 day before') subscribe('user1', 'topic2', '2025-12-09', '3 days before') send_schedule()