I think every programmer needs to know how Hash Function, and Hashpams/Tables work from the inside-out. It gives you a great insight on why certain techniques

I think every programmer needs to know how Hash Function, and Hashpams/Tables work from the inside-out. It gives you a great insight on why certain techniques are used, and it is a classical CS problem. If you can write a simple but solid implementation of a hash function, and a hash map, you are ahead of most developers out there. A lot of developers use them everyday, but don't have the faintest idea on how they work.