resizing tradeoffs when implementing Hashtable using linear probing

Question

I am trying to implement a hashtable using linear probing.

Before inserting a (key, value) pair into the hashtable, I want to check if it's half full. If it is, I need to double the size of the underlying array.

Obviously, there are two ways to do that:

One is to create another array with the doubled size, rehash all entries in the old one and add them to the new array. Then, rebind the old array to the new one. This way is easy to implement but uses a lot of space.

The other one is to double the array and do the rehashing in-place. It seems that this way may lead to longer running time because rehashing may cause collisions with both newly hashed slots and old slots.

Which way should I use?

Answer 1

Your second solution only saves space during the resize process if there is in fact room to expand the existing hash table in-place - I think the chances of that being the case for a large hash table are quite slim, so I would just go for your first solution.