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📜 WAYPOINT LESSON

A Debugging Method

⭐ beginner⏳ 15 min read📍 Lesson 55 of 281

Stress against a slow correct solution — the fastest bug finder you own.

The loop

  1. Reproduce on the smallest failing input. A bug that only shows at N = 100,000 usually shows at N = 7 once you know where to look.
  2. Isolate — print intermediate state (prefix sums, loop variables) or cut the program in half.
  3. Form a hypothesis, predict the output, run. Wrong prediction narrows the search space; right prediction means you found it.
  4. Fix the cause, not the symptom. Adding if (x == 42) special cases is how bugs become monsters.
  5. Re-test every old case plus the boundary that broke you.

Stress testing (the superpower)

Write a slow-but-obviously-correct solver plus a tiny random-input generator, then compare outputs in a loop:

gen > in.txt && slow < in.txt > slow.txt && fast < in.txt > fast.txt
diff slow.txt fast.txt  # mismatch = your minimal witness

Ten seconds of this beats an hour of staring. It finds the failing input FOR you.

Reading judge verdicts

  • WA on a small test: logic bug. On a big test only: overflow or complexity.
  • TLE: count your operations; print a counter if unsure.
  • RTE: array bounds, division by zero, stack depth.
  • WA only on samples with negatives: initialization (0 is not neutral for max).