方式一：
String轉(zhuǎn)為NSString，再進(jìn)行截取稳摄。
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(lèi)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
