宣告字典

字典是鍵和值的無序集合。值與唯一鍵相關，並且必須屬於同一型別。

初始化 Dictionary 時，完整語法如下：

var books : Dictionary<Int, String> = Dictionary<Int, String>()

雖然更簡潔的初始化方式：

var books = [Int: String]()
// or
var books: [Int: String] = [:]

通過在逗號分隔列表中指定金鑰和值來宣告字典。可以從鍵和值的型別推斷出型別。

var books: [Int: String] = [1: "Book 1", 2: "Book 2"]
//books = [2: "Book 2", 1: "Book 1"] 
var otherBooks = [3: "Book 3", 4: "Book 4"]
//otherBooks = [3: "Book 3", 4: "Book 4"]