Design a hit counter that counts the number of hits received in the past 5 minutes (300 seconds).
- `hit(timestamp)` records a hit at the given timestamp (in seconds).
- `getHits(timestamp)` returns the number of hits in the past 300 seconds. A hit at time `t` counts toward a query at `timestamp` if `t > timestamp - 300` (the trailing 300-second window, exclusive of the boundary).
Hits and queries are issued in non-decreasing timestamp order.
Implement the function `hitCounter(operations)` where `operations` is a list of `[op, ...args]`: `["hit", timestamp]` and `["getHits", timestamp]`. For `"hit"` return `null`; for `"getHits"` return the count. Return the list of results, one per operation (use null for operations that return nothing).