Relief Courier Queue

Problem

A relief organization hires local couriers for individual deliveries. Design ReliefCourierQueue to record each available courier's quoted fee and dispatch the lowest-cost available courier.

Requirements

Empty start. Construction starts with no available couriers.
Post courier. postCourier adds the courier ID and fee to the available couriers.
Fee-priority dispatch. dispatch removes and returns the available courier whose fee is smallest.
Availability lookup. isAvailable returns whether the courier ID is present.

API

Signature	Returns	Behavior
`ReliefCourierQueue()`	Not applicable	Creates an empty queue.
`postCourier(courierId: string, fee: integer)`	`void`	Adds the courier and its fee.
`dispatch()`	`string`	Removes and returns the lowest-fee available courier.
`isAvailable(courierId: string)`	`boolean`	Reports current availability.

Examples

Step	Operation	Result
1	`ReliefCourierQueue()`	Queue created
2	`postCourier("RIDGE", 70)`	No return value
3	`postCourier("VALLEY", 20)`	No return value
4	`dispatch()`	`"VALLEY"`
5	`isAvailable("VALLEY")`	`false`
6	`isAvailable("RIDGE")`	`true`

Constraints

At most 12 couriers are posted and at most 30 public method calls occur.
Courier IDs match [A-Za-z0-9_-]{1,24}.
1 <= fee <= 1000.
Courier IDs and fees are unique.
dispatch is called only when at least one courier is available.
Arguments are not null.

Notes

Calls are sequential.

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