I was recently asked this question in an interview:
Given a log file which has customer id and corresponding to that id it has a page id visited by that customer. Given such log files for 3 consecutive days, design an algo to find those customers who visited the site on exactly 2 out of 3 days and visited at least 3 distinct pages. Discuss the design and space complexity.

I thought of this approach:
1)sorting the 3 log files
2) looking through the top of each file: look for the same customer ids and see the number of different days they visited the pages. If the same customer has visited the pages on 3 different days, then we can discard that customer and move on looking to the next customer in each file. Else, if this customer has visited the the pages on exactly 2 different days, we will keep storing his page ids in a set.

But, I am not sure whether this approach is the most efficient, also what will be the approach which will scale well if I have to find k visited sites out of K sites, visiting exactly N distinct pages. I thought that k-d trees(2-d trees) may help because they can handle multiple queries.

So, what will be the best data strucuture and the algorithm to handle these kinds of queries.