Manacher算法(求最长回文字符串长度)

深碍√TFBOYSˉ_ 2022-05-26 04:00 258阅读 0赞

![这里写图片描述][70]

//
    public class StringProblem{ 
    
         //Manacher算法 预处理
         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;
        }
       //Manacher算法 O(N)复杂度求最长回文子串
        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;
        }
    
        //添加最短字符串构成回文串，求添加的最短字符串
        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){
    
             //System.out.println("Hello");
            String str1 = "abc1234321ab";
            System.out.println(maxLcpsLength(str1)); //最长回文串的长度
    
            String str2 = "abcd123321";
            System.out.println(shortestEnd(str2));  //返回添加的获得最短字符串
    
         }
    
    }

![这里写图片描述][70 1]

[70]: /images/20220526/a50715262bc8446cb4907c5796053ee9.png
[70 1]: /images/20220526/1800bc78ad8044a2b693e7e9176a8068.png