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package g1801_1900.s1845_seat_reservation_manager;

// #Medium #Design #Heap_Priority_Queue #Programming_Skills_II_Day_17
// #2022_05_07_Time_47_ms_(87.63%)_Space_107.1_MB_(45.03%)

import java.util.PriorityQueue;
import java.util.Queue;

/**
 * 1845 - Seat Reservation Manager\.
 *
 * Medium
 *
 * Design a system that manages the reservation state of `n` seats that are numbered from `1` to `n`.
 *
 * Implement the `SeatManager` class:
 *
 * *   `SeatManager(int n)` Initializes a `SeatManager` object that will manage `n` seats numbered from `1` to `n`. All seats are initially available.
 * *   `int reserve()` Fetches the **smallest-numbered** unreserved seat, reserves it, and returns its number.
 * *   `void unreserve(int seatNumber)` Unreserves the seat with the given `seatNumber`.
 *
 * **Example 1:**
 *
 * **Input** ["SeatManager", "reserve", "reserve", "unreserve", "reserve", "reserve", "reserve", "reserve", "unreserve"] [[5], [], [], [2], [], [], [], [], [5]]
 *
 * **Output:** [null, 1, 2, null, 2, 3, 4, 5, null]
 *
 * **Explanation:** 
 *
 * SeatManager seatManager = new SeatManager(5); // Initializes a SeatManager with 5 seats. 
 *
 * seatManager.reserve(); // All seats are available, so return the lowest numbered seat, which is 1. 
 *
 * seatManager.reserve(); // The available seats are [2,3,4,5], so return the lowest of them, which is 2. 
 *
 * seatManager.unreserve(2); // Unreserve seat 2, so now the available seats are [2,3,4,5]. 
 *
 * seatManager.reserve(); // The available seats are [2,3,4,5], so return the lowest of them, which is 2. 
 *
 * seatManager.reserve(); // The available seats are [3,4,5], so return the lowest of them, which is 3. 
 *
 * seatManager.reserve(); // The available seats are [4,5], so return the lowest of them, which is 4. 
 *
 * seatManager.reserve(); // The only available seat is seat 5, so return 5. 
 *
 * seatManager.unreserve(5); // Unreserve seat 5, so now the available seats are [5].
 *
 * **Constraints:**
 *
 * *   1 <= n <= 10⁵
 * *   `1 <= seatNumber <= n`
 * *   For each call to `reserve`, it is guaranteed that there will be at least one unreserved seat.
 * *   For each call to `unreserve`, it is guaranteed that `seatNumber` will be reserved.
 * *   At most 10⁵ calls **in total** will be made to `reserve` and `unreserve`.
**/
@SuppressWarnings("java:S1172")
public class SeatManager {
    private final Queue seats;
    private int smallest;

    public SeatManager(int n) {
        seats = new PriorityQueue<>();
        smallest = 0;
    }

    public int reserve() {
        return seats.isEmpty() ? ++smallest : seats.poll();
    }

    public void unreserve(int seatNumber) {
        seats.offer(seatNumber);
    }
}