Flight Software applicants have rated the interview process at SpaceX with 3 out of 5 (where 5 is the highest level of difficulty) and assessed their interview experience as 100% positive. To compare, the company-average is 53.4% positive. This is according to Glassdoor user ratings.
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Applied online and contacted recruiter. Communicated by email for a few weeks, with some large gaps. Had a phone call with them asking some basic introductory and behavioral questions. Then had a technical phone screen with an engineer, who asked mostly C++ and data structures questions.