Given a string of digits from 2 to 9 and the classic phone keypad mapping, return all possible letter combinations the number could represent.

"I interviewed for an SDE II role, and the whole process felt pretty standard and transparent. Amazon told me upfront what the process would look like, so I did not feel like I was guessing what came next: first an online assessment, then three 40-minute interviews back to back on the same day. The technical questions were all very classic problems, and the biggest surprise was that my interviewers often started with coding first instead of spending the first part on behavioral. The behavioral side was still important, but the follow-ups went deep into the actual details of my work instead of stopping at a polished STAR answer. After the loop, my recruiter mostly asked about logistics like start date and work authorization, and then they sent me the offer letter."