Charge each request only once
The payment webhook is delivered at least once, sometimes more. Each delivery carries an idempotency key, and a replay must not charge the customer twice.
- Sum the amounts, but only the first event carrying a given key counts.
- A later event with a key already seen is a replay, whatever amount it claims.
applyOnce(events: list<Event>) → int
Java needs a compiler and Drill does not host one yet, so this page is the reference rather than an exercise: the problem, worked examples, and the solution in full. To type it out, the same problem runs in Python.
Where you start
int applyOnce(List<Event> events) {
}Worked examples
| Call | Result |
|---|---|
applyOnce(Main.<Event>ls(new Event("a", 100), new Event("b", 50), new Event("a", 100))) | 150 |
applyOnce(Main.<Event>ls(new Event("a", 100), new Event("a", -100))) | 100 |
applyOnce(Main.<Event>ls(new Event("x", 7))) | 7 |
applyOnce(Main.<Event>ls()) | 0 |
Hint
Remember the keys you have already honoured.
Reference solution in Java
int applyOnce(List<Event> events) {
Set<String> seen = new HashSet<>();
int total = 0;
for (Event e : events) {
if (!seen.add(e.key)) continue;
total += e.amount;
}
return total;
}