An internal recruiter contacted me. I was looking for a new role at the time. (They had contacted me a year earlier, but I wasn't looking then.) The interview process consisted of one phone screen followed by an all-day on-site interview. This included a mixture of Q&A and coding sessions, as well as a lunch break with a team member.

I was asked for an algorithm to connect credit cards and merchants into sets. This is a significant problem if you cannot keep everything in memory. I made fair progress but didn't finish everything. It was an interesting question, related to Square's business rather than a purely theoretical one. The interviewer kept modifying my code style, which seemed a little odd.