1845. Seat Reservation Manager

Problem

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 <= 10^5

• 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 10^5 calls in total will be made to reserve and unreserve.

Solution (Java)

class SeatManager {
    private final Queue<Integer> 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);
    }
}

/**
 * Your SeatManager object will be instantiated and called as such:
 * SeatManager obj = new SeatManager(n);
 * int param_1 = obj.reserve();
 * obj.unreserve(seatNumber);
 */

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).