Meta Research Scientist I interview questions

I received two coding questions to solve. Both problems could be at a medium difficulty level on LeetCode. For the first question, the Meta interviewer started with a scenario where you are given two long arrays which are too big to be loaded into RAM and yet you want to compute the dot product of them. How would you solve this problem? So you need to come up with a data structure and algorithm to efficiently load these long arrays and compute the sum, and then write code for your solution to this problem. And the second coding problem is about the binary tree depth-first search.