Describe an efficient multimap structure for storing n entries that have an associated set of r < n keys that come from a total order. That is, the set of keys is smaller than the number of entries. Your structure should perform operation getAll in O(logr+s) expected time, where s is the number of entries returned, operation entrySet() in O(n) time, and the remaining operations of the multimap ADT in O(logr) expected time.