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
C# 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
public int ApplyOnce(List<Event> events) {
}Worked examples
| Call | Result |
|---|---|
ApplyOnce(new List<Event> { new Event("a", 100), new Event("b", 50), new Event("a", 100) }) | 150 |
ApplyOnce(new List<Event> { new Event("a", 100), new Event("a", -100) }) | 100 |
ApplyOnce(new List<Event> { new Event("x", 7) }) | 7 |
ApplyOnce(new List<Event> { }) | 0 |
Hint
Remember the keys you have already honoured.
Reference solution in C#
public int ApplyOnce(List<Event> events) {
var seen = new HashSet<string>();
int total = 0;
foreach (var e in events) {
if (!seen.Add(e.Key)) continue;
total += e.Amount;
}
return total;
}