最长回文（manacher模板）

#include<iostream>
#include<algorithm>
#include<cstring>
#include<cstdio>
using namespace std;
const int maxn = 2e5 + 1000;
char s[maxn], str[maxn];
int len1, len2, p[maxn], ans;

void init(){
    str[0] = '#';    str[1] = '#';
    for (int i = 0; i<len1; ++i)
    {
        str[i * 2 + 2] = s[i];
        str[i * 2 + 3] = '#';
    }
    len2 = len1 * 2 + 2;
    str[len2] = '*';
}

void manacher(){
    int id = 0, mx = 0;
    for (int i = 1; i < len2; ++i){
        if (mx>i)p[i] = min(p[id * 2 - i], mx - i);
        else p[i] = 1;
        for (; str[i+p[i]] == str[i - p[i]]; ++p[i]);
        if (p[i] + i > mx){ mx = p[i] + i; id = i; }
    }
}

int main(){
    while (~scanf("%s", s)){
        len1 = strlen(s);
        init();
        manacher();
        ans = 0;
        for (int i = 1; i < len2; ++i)
            ans = max(ans, p[i]);
        cout << ans - 1 << endl;
    }
}