Leetcode 381. Insert Delete GetRandom O(1) - Duplicates allowed

Design a data structure for a multiset that supports insert(val) (adding even duplicate occurrences), remove(val) (removing a single occurrence), and getRandom() (returning an element with probability proportional to its count), all in average O(1) time. The core challenge is handling duplicates and enabling O(1) arbitrary-removal while preserving uniform random access.