[国家集训队]最长双回文串 （PAM）回文自动机

Code:

// luogu-judger-enable-o2
#include <cstdio>
#include <algorithm>
#include <cstring>
#define maxn 300005
#define setIO(s) freopen(s".in","r",stdin)
using namespace std;
char str[maxn]; 
int n,l[maxn],ans,r[maxn];  
struct PAM{
    int fail[maxn],ch[maxn][30],len[maxn],tot,ss[maxn],last; 
    void init(){
        ss[0] = -1,fail[0] = 1,last = 0,fail[1] = 0; 
        len[0] = 0,len[1] = -1, tot = 1; 
    }
    int newnode(int x){ len[++tot] = x; return tot;  }
    int getfail(int p,int nn){
        while(ss[nn - len[p] - 1] != ss[nn]) p = fail[p];
        return p; 
    }
    void ins(int c,int i){
        int p = getfail(last,i);  
        if(!ch[p][c]) {
            int q = newnode(len[p] + 2); 
            fail[q] = ch[getfail(fail[p],i)][c];
            ch[p][c] = q; 
        }
        last = ch[p][c]; 
    }
}A,B;
int main(){
    //setIO("input");
    scanf("%s",str+1),n = strlen(str + 1), A.init(),B.init();
    for(int i = 1;i <= n; ++i) A.ss[i] = str[i] - 'a';
    for(int i = n;i >= 1; --i) B.ss[n - i + 1] = str[i] - 'a'; 
    for(int i = 1;i <= n; ++i) 
    {
        A.ins(A.ss[i],i);    
        l[i] = A.len[A.last];  
    }
    for(int i = 1;i <= n; ++i) B.ins(B.ss[i],i),r[n - i + 1] = B.len[B.last]; 
    for(int i = 1;i <= n - 1; ++i) ans = max(ans,l[i] + r[i + 1]); 
    printf("%d",ans); 
    return 0; 
}