Problem (verbatim):
“A multiset (also known as a bag) is a mutable, unordered collection of distinct objects that may appear more than once in the collection.
Implement a multiset that implements the following methods:
• add(element)
• remove(element)
• count(for: element)”
English summary (concise approach):
Back the multiset with a hash map value → frequency.
add(e): increment frequency.remove(e): decrement if present; delete key at zero (ignore if absent unless spec says otherwise).count(e): return frequency or 0.
All ops are amortized O(1); space is O(#unique).
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