最长回文子字符串

 https://www.jianshu.com/p/116aa58b7d81

https://zhuanlan.zhihu.com/p/70532099

https://leetcode-cn.com/problems/longest-palindromic-substring/solution/zhong-xin-kuo-san-dong-tai-gui-hua-by-liweiwei1419/

在字符串每个字符间加#，使字符串变为2n+1+1为奇数长度

再以newStr的每一个字符为中心向两边扩散，马拉车算法见上面链接

@Test
    public void test() {
        String babad = longestPalindrome("babadada");
        System.out.println("babadada = " + babad);
    }
    //#b#a#b#a#d#
    public String longestPalindrome(String s) {
        StringBuilder sb = new StringBuilder("#");
        char[] chars = s.toCharArray();
        for (char c : chars) {
            sb.append(c).append("#");
        }
        int maxlen = 0;
        int c = -1;
        int r = -1;
        String newStr = sb.toString();
        int[] arr = new int[newStr.length()];
        String res = "";
        for (int i=0;i<newStr.length();i++) {
            arr[i] = r > i ? Math.min(r-i+1, arr[2 * c - i]) : 1;
            while (i + arr[i] < newStr.length() && i - arr[i] > -1) {
                if (newStr.charAt(i-arr[i]) == newStr.charAt(i+arr[i])){
                    arr[i]++;
                } else {
                    break;
                }
            }
            if (i + arr[i] > r) {
                r = i + arr[i] - 1;
                c = i;
            }
            if (arr[i] > maxlen) {
                maxlen = arr[i];
                res = newStr.substring(i-arr[i]+1,i+arr[i]);
            }
        }


        // for (int i=0;i<newStr.length();i++) {
        //     for (int j=i-1,k=i+1,len = 1;j>=0 && k<newStr.length();j--,k++) {
        //         if (newStr.charAt(j) != newStr.charAt(k)) {
        //             break;
        //         }
        //         len += 2;
        //         if (len/2 > maxlen) {
        //             maxlen = len / 2;
        //             res = newStr.substring(j,k+1);
        //         }
        //     }
        // }
        return res.replace("#","");
    }