I met them at a university career fair and was invited to an on-campus interview the same week. There were three in-person interviews in total:

• The first interview involved basic (easy-medium) LeetCode-type questions.
• The second asked a design (OOP) question, and I was also asked to give a high-level overview of a solution to a harder algorithm question.
• In the third interview, I was asked to implement Karatsuba fast multiplication (with the optimization).

The first two interviews were manageable. The last interview was hard.

Invert (mirror) a binary tree (easy)

Given a list containing the centers (on the x-axis) and diameters of a number of balloons, determine the minimum number of darts you need to throw in order to pop all the balloons. You are throwing darts vertically, perpendicular to the x-axis.