String

    //string 和 Int的相互转换
    let numStr = "07d77"
    var n:Int = 0

    //String转Int
    if let number = Int(numStr){
        n = number
    }
    //Int转字符串
    print(String(n) + "jiji")
    //下面也可以
    let i = (str as NSString).intValue


    //截取字符串
    let subStr : String = (mStr as NSString).substring(with: NSRange(location: 2, length: 2));
    //PS：进行字符串操作时，截取等操作，String不如NSString方便，用as进行转换

    //循环遍历字符串，字符串倒置
    let mStr = "0123456789!"
    for charetder : Character in mStr.characters {
        print(charetder)
    }
    let revertStr = String(mStr.characters.reversed())
    print(revertStr)

    //字符串比较
    if str1 == str2 {
        print("equal")
    }

    if str.hasPrefix(str1) {
        print("has prefix")
    }

    if str.hasSuffix(str2) {
        print("has suffix")
    }

    //大小写转换
    let largeStr = str.uppercased()
    let xiaoStr = largeStr.lowercased()

    //输出utf8编码，0-255
    for codeUnit in str.utf8 {
        print(codeUnit)
    }