344ReverseString

      注意：String是字面量类型，即封装的字符数组内容不能改变，如要改变，只能重新生成新的字符串；此外，注意，数组下标的使用！

    public String reverseString(String s) { //literal type : String

        int len = s.length();

        char ch[] = s.toCharArray();

        for(int i = 0; i < len/2; i++){

            char tp;

            tp = ch[i];

            ch[i] = ch[len-1-i];

            ch[len-1-i] = tp;

        }

        //System.out.println(""+ s.charAt(0)+" "+ ch[0]);

        String rt = new String(ch);

        return rt;

    }