题目链接：https://www.luogu.com.cn/problem/P3805
manacher算法模板题。
参考资料：https://oi-wiki.org/string/manacher/
示例程序：
#include <bits/stdc++.h>
using namespace std;
const int maxn = 2.2e7 + 5;
int n;
char s[maxn/2], a[maxn];
int p[maxn];
void init() {
int m = 0;
a[m++] = '$';
a[m++] = '#';
for (int i = 0; i < n; i++)
a[m++] = s[i], a[m++] = '#';
a[m++] = '^';
n = m;
}
void manacher() {
int mr = 0, mid;
for (int i = 1; i < n; i++) {
if (i < mr) p[i] = min(p[mid * 2 - i], mr - i);
else p[i] = 1;
while (a[i - p[i]] == a[i + p[i]]) p[i]++;
if (i + p[i] > mr)
mr = i + p[i], mid = i;
}
}
int main() {
scanf("%s", s);
n = strlen(s);
init();
manacher();
int res = 0;
for (int i = 0; i < n; i++)
res = max(res, p[i]);
printf("%d\n", res - 1);
return 0;
}