11. Hash Tables

A hash table is a data structure that offers very fast insertion and searching. When you first hear about them, hash tables sound almost too good to be true. No matter how many data items there are, insertion and searching (and sometimes deletion) can take close to constant time: O(1) in big O notation. In practice this is just a few machine instructions.

For a human user of a hash table, this is essentially instantaneous. It’s so fast that computer programs typically use hash tables when they need to look up tens of thousands of items in less than a second (as in spelling checkers). ...

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