Manacher算法

短命女 2022-05-15 09:55 184阅读 0赞

# 一、[概要][Link 1] #

![这里写图片描述][70]

# 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() * 2 + 1];
            int index = 0;
            for (int i = 0; i != res.length; i++) {
                res[i] = (i & 1) == 0 ? '#' : charArr[index++];
            }
            return res;
        }
    
        public static int maxLcpsLength(String str) {
            if (str == null || str.length() == 0) {
                return 0;
            }
            char[] charArr = manacherString(str);
            int[] pArr = new int[charArr.length];
            int index = -1;
            int pR = -1;
            int max = Integer.MIN_VALUE;
            for (int i = 0; i != charArr.length; i++) {
                pArr[i] = pR > i ? Math.min(pArr[2 * index - i], pR - i) : 1;
                while (i + pArr[i] < charArr.length && i - pArr[i] > -1) {
                    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 - 1;
        }
    
        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() * 2 + 1];
            int index = 0;
            for (int i = 0; i != res.length; i++) {
                res[i] = (i & 1) == 0 ? '#' : charArr[index++];
            }
            return res;
        }
    
        public static String shortestEnd(String str) {
            if (str == null || str.length() == 0) {
                return null;
            }
            char[] charArr = manacherString(str);
            int[] pArr = new int[charArr.length];
            int index = -1;
            int pR = -1;
            int maxContainsEnd = -1;
            for (int i = 0; i != charArr.length; i++) {
                pArr[i] = pR > i ? Math.min(pArr[2 * index - i], pR - i) : 1;
                while (i + pArr[i] < charArr.length && i - pArr[i] > -1) {
                    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 + 1];
            for (int i = 0; i < res.length; i++) {
                res[res.length - 1 - i] = charArr[i * 2 + 1];
            }
            return String.valueOf(res);
        }
    
        public static void main(String[] args) {
            String str2 = "abcd123321";
            System.out.println(shortestEnd(str2));
    
        }
    
    }

[Link 1]: https://www.cnblogs.com/z360/p/6375514.html#undefined
[70]: /images/20220515/2bd033a9e6324226a66da7f77686f855.png