【BZOJ2160】拉拉队排练（回文树）

题面

题解

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<set>
#include<map>
#include<vector>
#include<queue>
using namespace std;
#define ll long long
#define RG register
#define MAX 1200000
#define MOD 19930726
int size[MAX];
char ch[MAX];
int n,ans=1;
ll K,sum;
pair<in
f47a
t,int> S[MAX];
int fpow(int a,int b)
{
int s=1;
while(b){if(b&1)s=1ll*s*a%MOD;a=1ll*a*a%MOD;b>>=1;}
return s;
}
struct PT
{
struct Node
{
int son[26];
int ff,len;
}t[MAX];
int last,tot;
void init()
{
t[tot=1].len=-1;
t[1].ff=t[0].ff=1;
}
void extend(int c,int n,char *s)
{
int p=last;
while(s[n-t[p].len-1]!=s
)p=t[p].ff;
if(!t[p].son[c])
{
int v=++tot,k=t[p].ff;
while(s[n-t[k].len-1]!=s
)k=t[k].ff;
t[v].len=t[p].len+2;
t[v].ff=t[k].son[c];
t[p].son[c]=v;
}
size[last=t[p].son[c]]++;
}
void Calc()
{
for(int i=tot;i;--i)size[t[i].ff]+=size[i];
}
}PT;
int main()
{
scanf("%d %lld",&n,&K);
scanf("%s",ch+1);
PT.init();
for(int i=1;i<=n;++i)PT.extend(ch[i]-97,i,ch);
PT.Calc();
for(int i=2;i<=PT.tot;++i)S[i-1]=make_pair(-PT.t[i].len,size[i]);
sort(&S[1],&S[PT.tot]);
for(int i=1;i<PT.tot;++i)
{
if(S[i].first%2==0)continue;
ll pl=min(K,1ll*S[i].second);
ans=1ll*ans*fpow(-S[i].first,pl)%MOD;
K-=pl;
if(!K)break;
}
if(!K)printf("%d\n",ans);
else puts("-1");
return 0;
}