Certainly. Of course, you should know how they work to the extent that you could write a (trivial) implementation if ever needed, but that very rarely happens.

Certainly. Of course, you should know how they work to the extent that you could write a (trivial) implementation if ever needed, but that very rarely happens. However, occasionally you do need to write specialised implementations tailored to your particular usecase and chosen data structures. Hash tables in particular come to mind. For example, I once had to write a statically allocated (fixed-size) hash table with O(1) lookups. A lot of tradeoffs enter the picture at that point so you had better know a dozen different types of hash tables to hash out a suitable implementation for the particular usecase.