[APIO2010] 回文串 - 回文自动机

经典题吧

我觉得我要换个板子，这结构体板子真TM不顺手

#include <bits/stdc++.h>
using namespace std;
const int N = 2e6 + 5;
struct PAM_Trie {
    int ch[26];
    int fail, len, cnt;
};
struct PAM {
    PAM_Trie b[N];
    int n, last, cnt, s[N];
    PAM() {
        b[0].len = 0;
        b[1].len = -1;
        b[0].fail = 1;
        b[1].fail = 0;
        last = 0;
        cnt = 1;
    }
    int get_fail(int x) {
        while(s[n - b[x].len - 1] != s[n]) {
            x = b[x].fail;
        }
        return x;
    }
    void insert(char ch) {
        s[++n]=ch;
        int p = get_fail(last);
        if(!b[p].ch[s[n]]) {
            b[++cnt].len = b[p].len + 2;
            int tmp = get_fail(b[p].fail);
            b[cnt].fail = b[tmp].ch[s[n]];
            b[p].ch[s[n]] = cnt;
        }
        last = b[p].ch[s[n]];
        b[last].cnt++;
    }
} P;
int length;
char c[N];
int main() {
    scanf("%s", c + 1);
    length = strlen(c + 1);
    int k=0;
    P.s[0]=26;
    for(int i=1;i<=length;i++) {
        P.insert(c[i]-'a');
    }
    long long mx=0;
    for(int i=P.cnt;i>=1;--i) {
        P.b[P.b[i].fail].cnt += P.b[i].cnt;
        mx=max(mx, (long long)P.b[i].len * P.b[i].cnt);
    }
    cout<<mx<<endl;
}