判断密码有效性

 int letter = 0;
    int digit = 0;
    int other = 0;
    
    for(NSUInteger i = 0; i < [text length]; i++) {
        int a = [text characterAtIndex:i];

        BOOL letterFlag = [[NSCharacterSet letterCharacterSet] characterIsMember:a];
        BOOL digitFlag = [[NSCharacterSet decimalDigitCharacterSet] characterIsMember:a];

        if (letterFlag) {
            letter = 1;
        }
        else if (digitFlag)
        {
            digit = 1;
        }
        else
        {
            other=1;
        }

    
    }
    return letter == 1 && (other + digit > 0) && text.length >= 6 && text.length <= 20;