Design Auction System solution and explanation

class AuctionSystem:

    def __init__(self):
        self.items = defaultdict(SortedList)
        self.users = {}

    def addBid(self, userId: int, itemId: int, bidAmount: int) -> None:
        if userId not in self.users:
            self.users[userId] = {}
        if itemId in self.users[userId]:
            self.removeBid(userId, itemId)
        self.users[userId][itemId] = bidAmount
        self.items[itemId].add((bidAmount, userId))

    def updateBid(self, userId: int, itemId: int, newAmount: int) -> None:
        oldAmount = self.users[userId][itemId]
        self.items[itemId].remove((oldAmount, userId))
        self.items[itemId].add((newAmount, userId))
        self.users[userId][itemId] = newAmount

    def removeBid(self, userId: int, itemId: int) -> None:
        oldAmount = self.users[userId][itemId]
        self.items[itemId].remove((oldAmount, userId))
        self.users[userId].pop(itemId)

    def getHighestBidder(self, itemId: int) -> int:
        ls = self.items[itemId]
        return -1 if not ls else ls[-1][1]


# Your AuctionSystem object will be instantiated and called as such:
# obj = AuctionSystem()
# obj.addBid(userId,itemId,bidAmount)
# obj.updateBid(userId,itemId,newAmount)
# obj.removeBid(userId,itemId)
# param_4 = obj.getHighestBidder(itemId)

class AuctionSystem {
    private final Map<Integer, TreeSet<int[]>> items = new HashMap<>();
    private final Map<Integer, Map<Integer, Integer>> users = new HashMap<>();

    public AuctionSystem() {
    }

    public void addBid(int userId, int itemId, int bidAmount) {
        users.computeIfAbsent(userId, k -> new HashMap<>());

        if (users.get(userId).containsKey(itemId)) {
            removeBid(userId, itemId);
        }

        users.get(userId).put(itemId, bidAmount);

        items.computeIfAbsent(itemId, k -> new TreeSet<>(
            (a, b) -> a[0] != b[0] ? Integer.compare(a[0], b[0]) : Integer.compare(a[1], b[1])
        ));

        items.get(itemId).add(new int[]{bidAmount, userId});
    }

    public void updateBid(int userId, int itemId, int newAmount) {
        int oldAmount = users.get(userId).get(itemId);
        TreeSet<int[]> set = items.get(itemId);

        set.remove(new int[]{oldAmount, userId});
        set.add(new int[]{newAmount, userId});

        users.get(userId).put(itemId, newAmount);
    }

    public void removeBid(int userId, int itemId) {
        int oldAmount = users.get(userId).get(itemId);
        TreeSet<int[]> set = items.get(itemId);

        set.remove(new int[]{oldAmount, userId});
        users.get(userId).remove(itemId);
    }

    public int getHighestBidder(int itemId) {
        TreeSet<int[]> set = items.get(itemId);
        if (set == null || set.isEmpty()) {
            return -1;
        }
        return set.last()[1];
    }
}

/**
 * Your AuctionSystem object will be instantiated and called as such:
 * AuctionSystem obj = new AuctionSystem();
 * obj.addBid(userId,itemId,bidAmount);
 * obj.updateBid(userId,itemId,newAmount);
 * obj.removeBid(userId,itemId);
 * int param_4 = obj.getHighestBidder(itemId);
 */

class AuctionSystem {
    unordered_map<int, set<pair<int, int>>> items;
    unordered_map<int, unordered_map<int, int>> users;

public:
    AuctionSystem() {
    }

    void addBid(int userId, int itemId, int bidAmount) {
        if (users[userId].count(itemId)) {
            removeBid(userId, itemId);
        }
        users[userId][itemId] = bidAmount;
        items[itemId].insert({bidAmount, userId});
    }

    void updateBid(int userId, int itemId, int newAmount) {
        int oldAmount = users[userId][itemId];
        auto& s = items[itemId];
        s.erase({oldAmount, userId});
        s.insert({newAmount, userId});
        users[userId][itemId] = newAmount;
    }

    void removeBid(int userId, int itemId) {
        int oldAmount = users[userId][itemId];
        auto& s = items[itemId];
        s.erase({oldAmount, userId});
        users[userId].erase(itemId);
    }

    int getHighestBidder(int itemId) {
        auto it = items.find(itemId);
        if (it == items.end() || it->second.empty()) {
            return -1;
        }
        return it->second.rbegin()->second;
    }
};

/**
 * Your AuctionSystem object will be instantiated and called as such:
 * AuctionSystem* obj = new AuctionSystem();
 * obj->addBid(userId,itemId,bidAmount);
 * obj->updateBid(userId,itemId,newAmount);
 * obj->removeBid(userId,itemId);
 * int param_4 = obj->getHighestBidder(itemId);
 */