Like I said to the other guy, depends on what you're working on. If you're building a product for users, there's a chance you have never implemented this. If you're building a product for developers, those chances should be lower. For example, I doubt anyone writing business logic for an API thought "time to implement a linked list!" instead of just importing a built-in linked list or a third-party lib.

Someone writes those libraries. Not you, apparently.