class Solution {private:    const char sep_char     = ‘\1‘;public:    string longestPalindrome(string s) {        int max_len = 0;        int len = s.length();        if (len <= 1) return s;                string str;        str.push_back(sep_char);        for (int i=0; i<len; i++) {            str.push_back(s[i]);            str.push_back(sep_char);        }        int nlen = 2 * len + 1;        vector<int> P(nlen, 0);                int last_i = 0;        int last_r = 0;                int maxv = -1;        int maxi = -1;        for (int i=0; i<nlen; i++) {            int p = i, q = i;            if (i < last_r) {                int j = 2 * last_i - i;     // (i + j) / 2 = last_i                int slen = min(P[j], last_r - i);                p-= slen;                q+= slen;            }                        while(p >= 0 && q < nlen && str[p] == str[q]) p--, q++;            if (q > last_r) {                last_r = q;                last_i = i;            }            P[i] = q - i;                        if (P[i] > maxv) {                maxv = P[i];                maxi = i;            }        }        return s.substr((maxi + 1 - P[maxi]) / 2, P[maxi] - 1);    }};