public String longestNiceSubstring(String s) {
        int n = s.length();
        int begin = 0;//最长子串开始
        int end = 0;//最长子串结束
        //枚举子串
        for (int i = 0; i < n; ++i) {
            int lower = 0;//二进制记录字母大小写出现结果
            int upper = 0;
            for (int j = i; j < n; ++j) {
                if (Character.isLowerCase(s.charAt(j))) {
                    lower |= 1 << (s.charAt(j) - 'a');
                } else {
                    upper |= 1 << (s.charAt(j) - 'A');
                }
                //两者相等表示大小写都有
                if (lower == upper && j - i  > end-begin) {
                    begin = i;
                    end = j;
                }
            }
        }
        if(begin-end==0)//找不到
            return "";
        return s.substring(begin, end+1);
    }