This was mostly done as a joke, but is also my first Android app.

Hilariously, the blog post http://www.joshuasnider.com/update/android/gaming/2015/08/17... I made about it has about 60% as many words as the program.

Did you implement minimax algorithm?

Yeah, the AI decides on the optimal move using a minmax algorithm. The exceptions are for the first move when its playing as X and when "easy mode" is enabled where it just plays randomly.