方式一：

String轉(zhuǎn)為NSString，再進(jìn)行截取。

        let string : String = "www.baidu.com"
        let resultStr = (string as NSString).substring(to: 3)

結(jié)果：www

方式二：

直接使用String類型方法進(jìn)行截取

let urlString : String = "www.baidu.com"
let headerIndex = urlString.index(urlString.startIndex, offsetBy: 3)
let header = urlString[..<headerIndex]

結(jié)果：www

let startIndex = urlString.index(urlString.startIndex, offsetBy: 4)
let endIndex = urlString.index(urlString.startIndex, offsetBy: 9)
let middle = urlString[startIndex..<endIndex]

結(jié)果:baidu

let footerIndex = urlString.index(urlString.endIndex, offsetBy: -3)
let footer = urlString[footerIndex..<urlString.endIndex]

結(jié)果:com