Swift声明字典
示例
字典是键和值的无序集合。值与唯一键有关，并且必须具有相同的类型。
初始化字典时，完整语法如下：
var books : Dictionary<Int, String> = Dictionary<Int, String>()
尽管初始化的方式更简洁：
var books = [Int: String]() //要么 var books: [Int: String] = [:]
通过在逗号分隔的列表中指定键和值来声明字典。可以从键和值的类型推断出类型。
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"]