[国家集训队]拉拉队排练 Manancher_前缀和_快速幂

Code:

#include <cstdio>
#include <algorithm>
#include <cstring>
using namespace std;
#define setIO(s) freopen(s".in","r",stdin)
#define maxn 2300000
#define mod 19930726
#define ll long long
char str[maxn],s[maxn];
int p[maxn],maxv;
ll arr[maxn],brr[maxn],k;
int Init(){
    int len=strlen(s),j=1;
    str[0]='$',str[1]='#';
    for(int i=0;i<len;++i)str[++j]=s[i],str[++j]='#';
    str[++j]='\0';
    return j;
}
void solve(){
    int len=Init(),id=1,mx=1;
    for(int i=1;i<len;++i){
        if(i<mx) p[i]=min(p[2*id-i],mx-i);
        else p[i]=1;
        while(str[i-p[i]]==str[i+p[i]]) ++p[i];
        if(p[i]+i>mx) mx=p[i]+i,id=i;
        if(str[i]!='#')
        {
            arr[1]+=1,arr[p[i]+1]-=1;
            maxv=max(maxv,p[i]);
        }
    }
}
ll qpow(ll a,ll p){
    ll res=1;
    while(p){
        if(p&1) res*=a,res%=mod;
        a*=a,a%=mod;
        p>>=1;
    }
    return res;
}
int main(){
    //setIO("input");
    int n;
    ll k,ans=1;
    scanf("%d%lld",&n,&k);
    scanf("%s",s);
    solve();
    for(int i=1;i<=maxv;++i) arr[i]+=arr[i-1];
    for(int i=1;i<=maxv;++i)
        if(i%2==0&&arr[i]) brr[i-1]=arr[i];
    for(int i=maxv;i>=1;--i){
        if(!arr[i]) continue;
        ans*=qpow(i,min(brr[i],k)),ans%=mod;
        //printf("%d %lld\n",i,ans);
        k-=brr[i];
        if(k<=0) break;
    }
    if(k>0) printf("-1");
    else printf("%lld",ans);
    return 0;
}