The Debugging Round

Somewhere in the full-day final loop, HRT stops asking you to write code and hands you code instead. Their own account of the day lists "coding and debugging rounds" side by side, and the debugging round is the one that sets the loop apart. You are dropped into a codebase you have never seen and asked to find what is broken and fix it, against the clock. Candidates report that you do this by connecting into a real development environment, reportedly over SSH, and working in the actual tree rather than a sanitized snippet on a shared editor.

That is a different skill from solving a fresh problem, and it deserves its own rehearsal. This lesson lays out the method first, then walks three defects end to end. Each is the kind of subtle bug this round favors: an arithmetic overflow that only shows on a large fill, a reference that goes stale when a container grows, and a variable nobody set. For each you get the code as it might be handed to you, the diagnosis spoken the way the round wants to hear it, and the corrected version.

The Hudson River Trading angle

Reading and fixing code you did not write, out loud, under time, is the daily job of a trading-systems engineer. A production trading system is decades of C++ that many hands have touched, and when a strategy misbehaves in the middle of the day nobody gets to start from a blank file. HRT built a round to test that skill directly, and it is the round a FAANG loop never runs. Prepare for it the way the job works: pull up code you did not write, break it on purpose, and practice narrating the hunt from symptom to fix.