Design and implement a simple rating system (like movie/book reviews) that supports adding ratings and querying summary statistics. #### Requirements Implement a data structure/class that supports the following operations: - `addRating(itemId, userId, score)`: user gives an item a numeric rating. - `updateRating(itemId, userId, newScore)`: user updates their rating (if it exists). - `removeRating(itemId, userId)`: user deletes their rating (if it exists). - `getItemStats(itemId)`: return at least: - total number of ratings - sum of scores - average score #### Notes - `score` is an integer in `[1, 5]`. - Multiple users can rate the same item, but each `(itemId, userId)` pair should have at most one active rating. - Aim for efficient updates and queries. You may assume item and user IDs are strings or integers. Provide the time/space complexity of each operation.