最长回文子串

最长回文子串

回文，亦称回环，是正读反读都能一样的字符串。例如“12321”、“abba”等。
现在给你一个字符串，请你找出其中长度最长的回文。

import java.util.Scanner;

public class Main {


    private static char[] getMatcher(String str) {
        char[] ret = new char[str.length() << 1 | 1];

        for (int i = 0; i < str.length(); ++i) {
            ret[i << 1] = '#';
            ret[i << 1 | 1] = str.charAt(i);
        }

        ret[ret.length - 1] = '#';

        return ret;
    }

    private static String solve(String str) {
        if (str == null || str.length() == 0) {
            return "";
        }

        char[] matcher = getMatcher(str);

        int c = -1, r = -1, max = 0, maxIndex = -1;
        int[] p = new int[matcher.length];

        for (int i = 0; i < matcher.length; ++i) {
            // 0 1 2 3
            p[i] = i > r ? 1 : Math.min(r - i + 1, p[2 * c - i]);
            while (i + p[i] < matcher.length && i - p[i] >= 0) {
                if (matcher[i + p[i]] == matcher[i - p[i]]) {
                    p[i]++;
                } else {
                    break;
                }
            }
            if (i + p[i] - 1 > r) {
                r = i + p[i] - 1;
                c = i;
            }
            if (p[i] > max) {
                max = p[i];
                maxIndex = i;
            }
        }

        // #a#b#a#
        // #a#a#
        int start = (maxIndex - max + 1) >> 1, end = start + max - 1;
        return str.substring(start, end);
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        while (in.hasNext()) {
            System.out.println(solve(in.next()));
        }
    }
}