> The most frequent data structure I've used regularly was hashtables and the hashing function. It's such a handy tool from counting, to detecting duplications,

> The most frequent data structure I've used regularly was hashtables and the hashing function. It's such a handy tool from counting, to detecting duplications, to caching, all the way to distributed systems use cases like sharding. After arrays, it's easily the most common data structure I've used on countless occasions. Almost all languages come with this data structure, and it's simple to implement if you'd need it. I'm surprised this wasn't listed first. I always ask a question or two related to hash tables when I interview candidates and I'm continuously shocked that a good chunk of candidates don't know the basics of hash tables. I expect you to know that looking up whether an item exists in a hash table is _generally_ fast in practice, even if you don't know the specifics of O(1) best case, O(n) worst case. I expect you to know that it doesn't allow duplicate keys. I don't expect you to implement a hash data structure from scratch, but I do expect you to know which data structure implements a hash table in your language of choice (e.g. Object, Dictionary, Map, etc.). It's the one slightly more complex "data structure" that I use all the time, so it's surprising to me when people don't know the basics.