The Beginner Bug Zoo
Five families cause most WA: boundaries, initialization, overflow, comparisons, state.
1. Boundary bugs
< vs <=, n-1 vs n, 0-based vs 1-based. Symptom: wrong on the
smallest or largest input only. Test: N = 1 and N = max, always.
2. Initialization bugs
max = 0 with all-negative input; forgetting to reset a counter
between test cases. Symptom: correct except on extreme values or
later cases.
3. Overflow bugs
int a = 100000; long long s = a * a; — the product happens in int
FIRST. Symptom: WA on big tests only, garbage-looking numbers.
4. Comparison bugs
Wrong comparator direction (sorts descending when you need ascending)
or <= where strictness matters (ties counted twice). Symptom:
almost-right order, or off-by-small amounts.
5. State bugs
Missing visited reset between queries, mutating shared data during
iteration, forgetting the undo in backtracking. Symptom: first query
correct, later ones garbage — or only the first test passes.
The fix discipline
After ANY fix: re-run every test you have, plus the one that failed. Two bugs often hide in one wrong program — fixing the first can change the symptoms of the second.