标识符：?和！
声明格式：
var str:String? ->可选类型 可以=nil
var str:String! ->隐式可选类型 可以=nil
var str:String = "stirng" ->非可选类型 不可以=nil
例1：
var str = "Hello, playground"
str = nil //编译报错：Nil cannot be assigned to type 'String'
var str: String = "Hello, playground"
str = nil //Nil cannot be assigned to type 'String'
例2：
var str: String! = "Hello, playground"
str = nil //编译通过 str == nil
print(str) //不必使用！
例3：
var str: String? = "Hello, playground"
print(str ?? " nil ")
print(str!)
