swift代码：思路是：反向读取+字符拼接

    func charAtReverse(_ str:String) -> String {
        var reverseStr:String = "";
        for i in 0..<str.count {
            let j = str.count-i-1;
            let currentIndex = str.index(str.startIndex, offsetBy:j);
            let currentStr = str[currentIndex]
            reverseStr.append(currentStr);
        }
        return reverseStr;
    }