【左神算法】Manacher问题

核心解决的是字符串中寻找最长回文问题

package com.ncst.improve.one;

/**
 * @author i
 * @create 2020/7/1 16:57
 * @Description 字符串最长回文问题
 */
public class Code_04_Manacher {

    //为字符串前后添加一个 # A#B#A
    public static char [] manacherString(String str){
        char[] chars = str.toCharArray();
        char [] res = new char [str.length()*2+1];
        int index = 0;
        for (int i = 0; i != res.length ; i++) {
            res[i] = (res[i]&1) == 0 ? '#' : chars[index++];
        }
        return res;
    }

    public static int maxLcpsLength(String str){
        if (str == null || str.length() == 0){
            return -1;
        }
        char [] chars = manacherString(str);
        int [] pArr = new int [chars.length];//回文半径数组
        int index = -1;
        int pR = -1;
        int max = Integer.MIN_VALUE;
        //遍历字符串
        for (int i = 0; i != chars.length ; i++) {
            //当前最右边界大于i  回文半径
            pArr[i] = pR > i ? Math.min(pArr[2 * index - i], pR - i) : 1;
            while (i + pArr[i] < chars.length && i - pArr[i] > -1) {
                if (chars[i + pArr[i]] == chars[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;
    }

}