Manacher算法

一、概要

二、算法实现

/**
 * 最大需要回文串的长度
 * @author superman
 *
 */
public class Manacher {

    public static char[] manacherString(String str) {
        char[] charArr = str.toCharArray();
        char[] res = new char[str.length() *  + ];
        int index = ;
        for (int i = ; i != res.length; i++) {
            res[i] = (i & ) ==  ? '#' : charArr[index++];
        }
        return res;
    }

    public static int maxLcpsLength(String str) {
        if (str == null || str.length() == ) {
            return ;
        }
        char[] charArr = manacherString(str);
        int[] pArr = new int[charArr.length];
        int index = -;
        int pR = -;
        int max = Integer.MIN_VALUE;
        for (int i = ; i != charArr.length; i++) {
            pArr[i] = pR > i ? Math.min(pArr[ * index - i], pR - i) : ;
            while (i + pArr[i] < charArr.length && i - pArr[i] > -) {
                if (charArr[i + pArr[i]] == charArr[i - pArr[i]])
                    pArr[i]++;
                else {
                    break;
                }
            }
            if (i + pArr[i] > pR) {
                pR = i + pArr[i];
                index = i;
            }
            max = Math.max(max, pArr[i]);
        }
        return max - ;
    }

    public static void main(String[] args) {
        String str1 = "abc1234321ab";
        System.out.println(maxLcpsLength(str1));
    }

}
           

/**
 * 返回添加的回文串
 * @author superman
 *
 */
public class Manacher_ShortestEnd {

    public static char[] manacherString(String str) {
        char[] charArr = str.toCharArray();
        char[] res = new char[str.length() *  + ];
        int index = ;
        for (int i = ; i != res.length; i++) {
            res[i] = (i & ) ==  ? '#' : charArr[index++];
        }
        return res;
    }

    public static String shortestEnd(String str) {
        if (str == null || str.length() == ) {
            return null;
        }
        char[] charArr = manacherString(str);
        int[] pArr = new int[charArr.length];
        int index = -;
        int pR = -;
        int maxContainsEnd = -;
        for (int i = ; i != charArr.length; i++) {
            pArr[i] = pR > i ? Math.min(pArr[ * index - i], pR - i) : ;
            while (i + pArr[i] < charArr.length && i - pArr[i] > -) {
                if (charArr[i + pArr[i]] == charArr[i - pArr[i]])
                    pArr[i]++;
                else {
                    break;
                }
            }
            if (i + pArr[i] > pR) {
                pR = i + pArr[i];
                index = i;
            }
            if (pR == charArr.length) {
                maxContainsEnd = pArr[i];
                break;
            }
        }
        char[] res = new char[str.length() - maxContainsEnd + ];
        for (int i = ; i < res.length; i++) {
            res[res.length -  - i] = charArr[i *  + ];
        }
        return String.valueOf(res);
    }

    public static void main(String[] args) {
        String str2 = "abcd123321";
        System.out.println(shortestEnd(str2));

    }

}