[回文自动机] BZOJ3676 [Apio2014] . 回文串

#include <cstdio>
#include <iostream>
#include <algorithm>
#include <cstring>

using namespace std;

const int N=300010;

int n,p,cnt=1;
int nxt
[30],fail
,len
,size
;
char a
;

inline void extend(int x,int pos){
while(a[pos]!=a[pos-len[p]-1]) p=fail[p];
if(!nxt[p][x]){
int cur=++cnt,k=fail[p]; len[cur]=len[p]+2;
while(a[pos]!=a[pos-len[k]-1]) k=fail[k];
fail[cur]=nxt[k][x]; nxt[p][x]=cur;
}
p=nxt[p][x]; size[p]++;
}

int main(){
fail[0]=fail[1]=1; len[1]=-1;
scanf("%s",a+1); n=strlen(a+1);
for(int i=1;i<=n;i++) extend(a[i]-'a',i);
long long ans=0;
for(int i=cnt;i;i--){
size[fail[i]]+=size[i];
ans=max(1LL*len[i]*size[i],ans);
}
printf("%lld\n",ans);
return 0;
}