问如何在Swift字典中为相同的值使用多个键？

EN

struct User {
    let userId: Int
    let firstName: String
    // more user properties
}

class UserDict {

    private var dict = [String: User]()

    subscript(key: String) -> User? {
        return dict[key]
    }

    subscript(key: Int) -> User? {
        return dict["\(key)"]
    }

    func add(_ user: User) {
        dict[user.firstName] = user
        dict["\(user.userId)"] = user
    }

    func remove(forKey key: String) {
        if let user = dict[key] {
            dict.removeValue(forKey: user.firstName)
            dict.removeValue(forKey: "\(user.userId)")
        }
    }

    func remove(forKey key: Int) {
        remove(forKey: "\(key)")
    }

    var count: Int {
        return dict.count / 2
    }
}