Swift-字符串转换成整数

题目：实现一个函数 stringToInt，实现把字符串转换成整数这个功能，不能使用系统提供的库函数.
题目很简单，不过需要考虑多种异常输入的情况，核心代码如下:

`
 func stringToInt(str:String?)->Int? {
 if str == nil || str?.characters.count == 0 {
 return nil
 }
 
 
    var result:Int = 0
    var begin:Int = 0
    let count:Int = str!.characters.count
    var flag:Int = 1
    
    let curStr:String = str![0]
    if curStr == "+" {
        flag = 1
        begin = 1
    } else if curStr == "-" {
        flag = -1
        begin = 1
    } else if !isNumberDigit(str: curStr) {
        return nil
    }
    
    
    for i in begin.. (Int.max - temp * flag) / 10 {
            return nil
        }
        
        if flag == -1 &&  result < (Int.min - temp * flag) / 10 {
            return nil
        }
        
        result = result * 10 + temp * flag
    }
    
    return result
}

func isNumberDigit(str:String)->Bool {
    return str >= "0" && str <= "9"
}`

测试代码:

var convert:Convert = Convert() var convertResult:Int? = convert.stringToInt(str: "-9987334") if convertResult != nil { print("FlyElephant--转换结果---||\(convertResult!)") } else { print("参数非法") }