Offer rescinded after accepting!

• 3 rounds of coding
• 1 design interview
• 1 manager interview

I obviously spent a lot of time and energy to go through all the rounds and was quite happy after negotiating and accepting an offer. But after a couple of weeks, they sent an email rescinding it, citing "business reasons". To make it worse, they didn't even have the decency to call and explain and simply "ghosted" me since. Very frustrating, obviously.

String-based coding questions and a standard system design question.