Collections - using hashmaps and hashsets

Often, you need to collect a large number of data items of a given type in one data structure. Until now, we have only worked with the Vec datatype to do this, but the std::collections module contains other implementations of sequences (such as LinkedList), maps (such as HashMap), and sets (such as HashSet). In most cases, you will only need Vec and HashMap; let's examine how to work with the latter.

A HashMap <K,V> is used when you want to store pairs consisting of a key of type K and a value of type V, both of generic type. K can be a Boolean, an integer, a string, or any other type that implements the Eq and Hash traits. A HashMap enables you to very quickly look up the value attached to a certain key using a hashing algorithm, and also because the data is cached. However, the keys of a HashMap are not sorted; if you need that, use a BTreeMap. HashMap is allocated on the heap, so it is dynamically resizable, just like a Vec.

Let's store monster...