manacher算法实现

 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;  //初始化  这里可以再优化 取最小的可能会增加次数
             while (i + pArr[i] < charArr.length && i - pArr[i] > -1) {  //是否越界
                 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]);  //这个数组可以变成一个map  这样不仅可以求出子串长度 还可以求出子串
         }

 

 return max - 1;

 

预处理串

 public 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;
     }