Where it comes from

How to answer

This is a standard sliding window in a sensor story. Solve it cleanly and spend the time you save on testing, because a small bug you can’t find costs more than a slow first draft. One candidate for a C3 AI role in Chicago reported, in August 2026, that their coding round on CodePair was a medium sliding-window and hash-map problem, and that a small indentation error they could not find in time ended the process. Source 1C3 AI - Forward Deployed Engineer interview process (comment by u/BalanceOdd144)PublisherReddit r/leetcodeSource typecandidate report on Reddit

  1. Pin the output. Start and end indexes, half-open, not just the length: an operator wants to know where it is. Agree on ties (first wins) and the empty log.
  2. Ask what “repeats” means for the values. Raw floats rarely repeat exactly, so ask whether readings are codes or need rounding. That changes the key, not the algorithm.
  3. Name the brute force, then the window. Checking every start is quadratic. Keep [left, right] free of repeats, with a map from each value to the last index it was seen. When the right edge meets a value last seen inside the window, move left to one past that index.
  4. Say the invariant and the guard. The map is never cleared, so an old entry can point before left. Move left only when the last position is at or after it.
  5. State the cost. One pass, linear time; the map holds one entry per distinct value.
  6. Run small cases early, and know what each one catches. Empty, all the same, all different, and a value that returns after its twin has already moved the window. All different catches the classic indentation slip, the best-length update tucked under the repeat branch: it returns an empty stretch. Then compare with the brute force on random logs.

The worst ten minutes on a production line drill pairs this window with a time-based one.

GlossaryForward deployed engineerA software engineer who builds and ships production systems inside a customer’s problem and environment, accountable to that customer’s outcome.More on Forward deployed engineer

Follow-ups

What the interviewer may ask next, once your first answer is on the table.

  • Each reading has a timestamp, and a stretch also ends at a gap of more than five minutes. What changes?
  • Allow each value to appear at most twice in a stretch. Does the last-position map still work?
  • The log is a stream you can’t hold in memory. What do you keep, and how large can it get?
  • Return every longest stretch, not just the first.

Where answers go wrong

  • Jumping the left edge to just after a value’s last position even when that position is already outside the window, which moves the window backwards and reports a stretch that contains a repeat.
  • Rebuilding a set from the left edge after every repeat, which is quadratic on logs with long clean stretches.
  • Returning only the length when the customer needs to know where the stretch is.

Answer this in two minutes

Write the answer you would say out loud. The clock starts with your first word.

Two minutes

Model answer

“I’ll return the stretch as (start, end), half-open, the first one on ties, and treat readings as exact values. You said they’re integer codes from the sensor, so no rounding.

Learn this in Module 6: Screens and unfamiliar code, part of the curriculum.