Google Software Engineer Internship interview questions

Q: given a valley with hills in each point of rights a1, a2, ..., an, determine how much water will be there in this valley after a rain.

I can't quite recall the coding questions I was asked, but I do know that data structures and algorithms are something that one needs to be aware of. I had not taken a course at my uni yet concerning the aspect at hand, but I had just basic knowledge about a few which was not enough I feel.