Background in telecommunications/signal processing (phd) + applied math. I studied finance + machine learning for two years as a hobby before the switch. In the evenings and weekends. Studied data structures + algorithms + python for two years before took a job as a software engineer in google (initially as a data scientist in apple). Worked 6 days a week for 1.5 yrs to master information retrieval in google. So lots of studying was involved on top of a solid educational base.