bzoj2565manacher算法

//先开始没发现*号的作用到后面发现阔以用*号来链接就好了。

//比如fff 转换后市#*f*f*f* reall[i]表示以i为起点往右最长的回文串长度，real[i]表示以i为起点往左的最长回文串长度

//当str[i]==‘*‘;的时ans=max(ans,reall[i]/2+real[i]/2);这样从左到右枚举*就好了。

//那么怎么求reall[i] (real[i]用manacher的时候就求了)从右到左枚举以某个点位为中心的最长回文串就是下面的代码

 int limitl = len - 1; reall[len - 1] = 1;
 for (int i = len - 1,k=len-1; i >= 1;i--)
 {
  if (limitl >i - pr[i] + 1)
  {
   k = limitl - 1;
   while (k >= i - pr[i] + 1)
   {
    reall[k] = max(reall[k],(i - k)*2+ 1);
    k--;
   }
   limitl = i - pr[i] + 1;
  }
 }

i就代表枚举的中心点其他自己看很好懂了。pr[i]是半径.*f*那么以f为中心的最长串半径是2

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<string.h>
using namespace std;
int id,len,ans;
int pr[300020];
int pl[300020];
char str[300020];
char in[200020];
int real[300020];
int reall[300020];
void manacher1()
{
	pr[1] = 1; id = 1;real[0] = -1000;
	//for (int i = 3; i < len; i++)
		//   real[i] = 1;
	for (int i = 2; i < len; i++)
	{
		if (id + pr[id]>i)
			pr[i] = min(id + pr[id] - i, pr[2 * id - i]);
		else
			pr[i] = 1;
		while (str[i + pr[i]] == str[i - pr[i]]){real[i + pr[i]] = max(real[i + pr[i]], 2 *pr[i] + 1); pr[i]++; }
		if (i + pr[i] > id + pr[id])id = i;
	}
}
void getans()
{
	int limitl = len - 1; reall[len - 1] = 1;
	for (int i = len - 1,k=len-1; i >= 1;i--)
	{
		if (limitl >i - pr[i] + 1)
		{
			k = limitl - 1;
			while (k >= i - pr[i] + 1)
			{
				reall[k] = max(reall[k],(i - k)*2+ 1);
				k--;
			}
			limitl = i - pr[i] + 1;
		}
	}
	for (int i = 2; i <= len - 2; i++)
	{
		if (str[i] == '*')
		{
			ans = max(ans, reall[i]/2 + real[i]/2);
		}
	}
}
int main()
{
	str[0] = '#';
	for (int i = 1;; i++)
	{
		str[i] = '*';
		i++;
		str[i] = getchar();
		if (str[i] == '\n')
		{
			str[i] = '\0';
			//str[i+1] = '\0';
			len = i;
			break;
		}
	}
	//str[1] = '&';
	//cout << str <<" "<<len<< endl;
	//cout << len << endl;
	if (len > 4)
	{
		ans = 2;
		manacher1();
		getans();
	}
	else
		ans = 0;
	printf("%d\n", ans);
	return 0;
}