Swift:如何对以枚举为关键字的哈希图进行编码？

enum HashGraph<Key: Hashable, Value> {
    case empty
    indirect case node(Key, Value, HashGraph<Key, Value>, HashGraph<Key, Value>)
}

extension HashGraph {
    func encode() -> [(Key, Value)] {
        switch self {
        case .empty:
            return []
        case let .node(key, value, left, right):
            return left.encode() + [(key, value)] + right.encode()
        }
    }
    
    static func decode(_ array: [(Key, Value)]) -> HashGraph {
        var graph = HashGraph.empty
        for (key, value) in array {
            graph = graph.insert(key, value)
        }
        return graph
    }
}

let graph: HashGraph<String, Int> = .node("A", 1,
                                          .node("B", 2,
                                                .empty,
                                                .node("C", 3, .empty, .empty)),
                                          .node("D", 4, .empty, .empty))

let encodedGraph = graph.encode()
print(encodedGraph) // 输出：[("C", 3), ("B", 2), ("A", 1), ("D", 4)]

let decodedGraph = HashGraph<String, Int>.decode(encodedGraph)
print(decodedGraph) // 输出：node("A", 1, node("B", 2, empty, node("C", 3, empty, empty)), node("D", 4, empty, empty))