public class Solution {
    // example in leetcode book
    public String longestPalindrome(String s) {
        int start = 0, end = 0;
        for (int i = 0; i < s.length(); i++) {
            // aba
            int len1 = expandAroundCenter(s, i, i);
            // bb
            int len2 = expandAroundCenter(s, i, i + 1);
            int len = Math.max(len1, len2);
            if (len > end - start) {
                start = i - (len - 1) / 2;
                end = i + len / 2;
            }
        }
        return s.substring(start, end + 1);
    }

    private int expandAroundCenter(String s, int left, int right) {
        int L = left, R = right;
        while (L >= 0 && R < s.length() && s.charAt(L) == s.charAt(R)) {
            L--;
            R++;
        }
        return R - L - 1;
    }



    // ManachersAlgorithm
    // https://en.wikipedia.org/wiki/Longest_palindromic_substring
    // public String longestPalindrome(String s) {
    //     if (s==null || s.length()==0)
    //         return "";

    //     char[] s2 = addBoundaries(s.toCharArray());
    //     int[] p = new int[s2.length];
    //     int c = 0, r = 0; // Here the first element in s2 has been processed.
    //     int m = 0, n = 0; // The walking indices to compare if two elements are the same
    //     for (int i = 1; i<s2.length; i++) {
    //         if (i>r) {
    //             p[i] = 0; m = i-1; n = i+1;
    //         } else {
    //             int i2 = c*2-i;
    //             if (p[i2]<(r-i)) {
    //                 p[i] = p[i2];
    //                 m = -1; // This signals bypassing the while loop below.
    //             } else {
    //                 p[i] = r-i;
    //                 n = r+1; m = i*2-n;
    //             }
    //         }
    //         while (m>=0 && n<s2.length && s2[m]==s2[n]) {
    //             p[i]++; m--; n++;
    //         }
    //         if ((i+p[i])>r) {
    //             c = i; r = i+p[i];
    //         }
    //     }
    //     int len = 0; c = 0;
    //     for (int i = 1; i<s2.length; i++) {
    //         if (len<p[i]) {
    //             len = p[i]; c = i;
    //         }
    //     }
    //     char[] ss = Arrays.copyOfRange(s2, c-len, c+len+1);
    //     return String.valueOf(removeBoundaries(ss));
    // }

    // private static char[] addBoundaries(char[] cs) {
    //     if (cs==null || cs.length==0)
    //         return "||".toCharArray();

    //     char[] cs2 = new char[cs.length*2+1];
    //     for (int i = 0; i<(cs2.length-1); i = i+2) {
    //         cs2[i] = '|';
    //         cs2[i+1] = cs[i/2];
    //     }
    //     cs2[cs2.length-1] = '|';
    //     return cs2;
    // }

    // private static char[] removeBoundaries(char[] cs) {
    //     if (cs==null || cs.length<3)
    //         return "".toCharArray();

    //     char[] cs2 = new char[(cs.length-1)/2];
    //     for (int i = 0; i<cs2.length; i++) {
    //         cs2[i] = cs[i*2+1];
    //     }
    //     return cs2;
    // }
}