bzoj 3676 [Apio2014]回文串

这道题在洛谷在洛谷也有（P3649）但数据不好。推荐到bzoj提交

回文自动机的模板题，求出PAM后统计即可。

代码：

#include<bits/stdc++.h>

using namespace std;

typedef long long ll;

const int maxn = 300000 + 5;

struct PAM
{
    int nxt[maxn][26], link[maxn], len[maxn], size[maxn], sz, last;
    char s[maxn];
    
    void inline extend(int c, int now)
    {
        int p = last;
        while(s[now - len[p] - 1] != s[now]) p = link[p];
        if(!nxt[p][c]) {
            int q = link[p];
            while(s[now - len[q] - 1] != s[now]) q = link[q];
            link[++sz] = nxt[q][c], nxt[p][c] = sz, len[sz] = len[p] + 2;
        }
        last = nxt[p][c]; ++size[last];
    }
    
    void inline build(char *str)
    {
        for(int i = 0; str[i]; ++i) s[i + 1] = str[i];
        link[0] = link[1] = 1; len[0] = 0, len[1] = -1;
        sz = 1;
        for(int i = 0; str[i]; ++i) extend(s[i + 1] - 'a', i + 1);
        for(int i = sz; i > 1; --i) size[link[i]] += size[i];
    }
} pam;

char str[maxn];

void inline Solve()
{
    ll ans = 0;
    pam.build(str);
    for(int i = 2; i <= pam.sz; ++i) ans = max(ans, (ll)pam.size[i] * pam.len[i]);
    printf("%lld\n", ans);
}

int main()
{
    scanf("%s", str);
    Solve();
    return 0;
}