Generate random string with provided number of characters

Random String with Count

class func genRandStringLength(len: Int) -> String {
    var randomString :String! = String();
        
    for index in 0..<len {
            
        let letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
        let randomIndex = arc4random_uniform(UInt32(countElements(letters)))
        let index = Int(randomIndex)
        let aChar = letters.characterAtIndex(index);
        randomString = randomString.stringByAppendingString(aChar);
    }
    return randomString
}