Seat Reservation Manager

Description

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.

Code

Min Heap

Time Complexity: $O(\log n)$, Space Complexity: $O(n)$

class SeatManager {
    priority_queue<int, vector<int>, greater<int>> seats; // min heap
public:
    SeatManager(int n) {
        for(int i = 1; i <= n; i++) {
            seats.push(i);
        }
    }
    
    int reserve() {
        auto seat = seats.top();
        seats.pop();
        return seat;
    }
    
    void unreserve(int seatNumber) {
        seats.push(seatNumber);
    }
};
 
/**
 * Your SeatManager object will be instantiated and called as such:
 * SeatManager* obj = new SeatManager(n);
 * int param_1 = obj->reserve();
 * obj->unreserve(seatNumber);
 */

Optimized Min Heap

和 Smallest Number in Infinite Set 中的解法一樣，不需要在一開始就將 $n$ 個數字都 push 到 heap 中。

class SeatManager {
    priority_queue<int, vector<int>, greater<int>> seats; // min heap
    int min_s = 0;
public:
    SeatManager(int n) {
    }
    
    int reserve() {
        auto seat = seats.empty() ? ++min_s : seats.top();
        if(!seats.empty()) seats.pop();
        return seat;
    }
    
    void unreserve(int seatNumber) {
        seats.push(seatNumber);
    }
};
 
/**
 * Your SeatManager object will be instantiated and called as such:
 * SeatManager* obj = new SeatManager(n);
 * int param_1 = obj->reserve();
 * obj->unreserve(seatNumber);
 */

Source

• Seat Reservation Manager - LeetCode