【BZOJ3676】[Apio2014]回文串 【回文自动机】

【题目链接】

学习一发回文自动机！

照着hzwer的模板打了一发

/* Pigonometry */
#include <cstdio>
#include <cstring>
#include <algorithm>

using namespace std;

typedef long long LL;

const int maxn = 300005;

char str[maxn];

struct _pam {
int son[maxn][26], fail[maxn], len[maxn], size[maxn], cnt, last;

void init() {
memset(son, 0, sizeof(son));
for(int i = 0; i < maxn; i++) fail[i] = len[i] = size[i] = 0;
fail[0] = fail[1] = 1;
len[0] = 0; len[1] = -1;
cnt = 1; last = 0;
}

void insert(int c, int n) {
int now = last;
for(; str[n - 1 - len[now]] != str
; now = fail[now]);
if(!son[now][c]) {
int pos = ++cnt;
len[pos] = len[now] + 2;
int k = fail[now];
for(; str[n - 1 - len[k]] != str
; k = fail[k]);
fail[pos] = son[k][c];
son[now][c] = pos;
}
last = son[now][c];
size[last]++;
}

LL solve() {
LL res = 0;
for(int i = cnt; i; i--) {
size[fail[i]] += size[i];
res = max(res, (LL)size[i] * len[i]);
}
return res;
}
} pam;

int main() {
scanf("%s", str + 1); int n = strlen(str + 1);
pam.init();
for(int i = 1; i <= n; i++) pam.insert(str[i] - 'a', i);
printf("%lld\n", pam.solve());
return 0;
}