P1659 [国家集训队]拉拉队排练

思路

求出cnt和len之后，直接乘起来即可

代码

#include <cstdio>
#include <algorithm>
#include <cstring>
#define int long long
using namespace std;
int Nodecnt,last,n,k;
signed trans[1001000][26],fail[1001000];
struct Node{
    signed len,cnt;
    bool operator < (const Node &b) const{
        return len>b.len;
    }
}a[1001000];
char s[1001000];
const int MOD = 19930726;
int pow(int a,int b){
    int ans=1;
    while(b){
        if(b&1)
            ans=(1LL*ans*a)%MOD;
        a=(1LL*a*a)%MOD;
        b>>=1;
    }
    return ans;
}
int New_state(int _len){
    a[Nodecnt].len=_len;
    return Nodecnt++;
}
int get_fail(int p,int n){
    while(s[n]!=s[n-a[p].len-1])
        p=fail[p];
    return p;
}
void insert(int n){
    int cur=get_fail(last,n);
    if(!trans[cur][s[n]]){
        int t=New_state(a[cur].len+2);
        fail[t]=trans[get_fail(fail[cur],n)][s[n]];
        trans[cur][s[n]]=t;
    }
    a[trans[cur][s[n]]].cnt++;
    last=trans[cur][s[n]];
}
signed main(){
    // freopen("test.in","r",stdin);
    int ans=1;
    scanf("%lld %lld",&n,&k);
    scanf("%s",s+1);
    s[0]=-1;
    New_state(0);
    fail[0]=1;
    New_state(-1);
    last=0;
    for(int i=1;i<=n;i++){
        s[i]-='a';
        insert(i);
    }
    for(int i=Nodecnt-1;i>=0;i--)
        a[fail[i]].cnt+=a[i].cnt;
    sort(a,a+Nodecnt);
    for(int i=0;i<Nodecnt&&k>0;i++){
        if(a[i].len%2){
            ans=(1LL*ans*pow(a[i].len,min(1LL*a[i].cnt,k)))%MOD;
            k-=a[i].cnt;
        }
    }
    printf("%lld\n",ans);
    return 0;
}