If you are targeting L4/E4 (3-6 years experience) at a top tech company, this PDF (specifically Alex Xu’s Volume 1 and 2) is the single best $0–$30 you will spend. It turns the terrifying "Design WhatsApp" question into a predictable, solvable puzzle.


If you want the content of the "Alex Wu PDF" without the guilt:

Pro tip: If you are a student, check your university library’s Springer or O’Reilly access. Many have it.


Alex Xu’s book specifically walks through 15 real-world questions. Here are the top 5 that you should practice, even without the PDF: