You have a coin that lands heads with some unknown probability p. Build a fair 50/50 flip from it without knowing p. The von Neumann trick is two lines; the part that scores is explaining why the bias cancels and how many flips it costs.
Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
You have a coin that lands heads with some unknown probability p. Build a fair 50/50 flip from it without knowing p. The von Neumann trick is two lines; the part that scores is explaining why the bias cancels and how many flips it costs.
This is a reasoning screen wearing a coding disguise. The interviewer is listening for the symmetry argument that P(HT) = P(TH), and the honest follow-up is the expected number of flips, which blows up as p approaches 0 or 1. Candidates who reach for entropy-extraction tricks before nailing the basic argument usually overcomplicate it.
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