Probability & Expected Value

On the trading and research tracks, Jane Street interviewers price games of chance with you and keep changing them. An interviewer asks what a die game is worth, waits for a number, then allows a second re-roll or tilts the coin in a random walk. A memorized answer covers the first question and nothing after it.

The method behind all of these is the same. Fix the boundary, condition on the next step and recurse. It's worked by hand further down, in Three moves that solve most of them. The four coded problems turn games from that round into functions you can run. Code forces every case of the recursion to exist, and it catches mistakes a mental answer hides, such as quitting one roll too early on a bank that sits next to a losing square.

All of it is for the trading and research tracks, where probability questions run from the phone screen through the onsite. Software candidates can skip it. Jane Street says it doesn't give engineers mental math, olympiad problems or logic puzzles.

Jane StreetPremium lesson · Coding round

The 4 Jane Street problems are below.

1. 1A Premium questionCoding · Medium
2. 2A Premium questionCoding · Medium
3. 3A Premium questionCoding · Hard

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