usage - "What I have to do is" + verb inf

I'm confused about the following examples, and I'm pretty sure I've seen both of them in books and white papers.

• What I have to do is find her.


• What I have to do is to find her.


• What I would like to do is to buy it.


• What I would like to do is buy it

Can anyone please tell me which one is correct?

Many thanks