swift 字符串截取

//swift 2.3之前可以使用
let hex = "112233"
//截取第二位到第四位
let greenHex = hex.substringWithRange(Range(start: hex.startIndex.advancedBy(2), end: hex.startIndex.advancedBy(4)))
但是swift3 会删除 start...end方法 提示报错  ('init(start:end:)' is deprecated: it will be removed in Swift 3. Use the '..<' operator)
在swift2.3可以使用
let myString = "112233"
截取第二到第四位
myString[myString.startIndex.advancedBy(2)..String.Index{
        return string.index(after: self)
    }
    
    func predecessor(in string:String)->String.Index{
        return string.index(before: self)
    }
    
    func advance(_ offset:Int, `for` string:String)->String.Index{
        return string.index(self, offsetBy: offset)
    }
}

//截取第二到第四位
let new = myString[myString.startIndex.advance(2, for: myString).. String {
        if self.characters.count