It's one thing to visualize, another to recall at will. Your friend is someone whom you've spent sufficient time with on at least a semi-regular basis to form emotional and mental bonds with, deep memories. How many times does a CS undergraduate re-implement sort algorithms? Not that most interviews actually ask one to regurgitate that from one's memory.
> How many times does a CS undergraduate re-implement sort algorithms
You don't have to implement, you just need to know the theory of why it works. Then you just code it up based on that theory, it isn't hard to do at all.
For example, merge sort explains itself, you partition and merge. That is all you need to remember. Hashmap is the same, you use a deterministic function to label objects and then put those into buckets, then you find them in the bucket with the same label later. These are like the basics of the basics.
