P5337 [TJOI2019]甲苯先生的字符串

#include<iostream>
#include<algorithm>
#include<cstring>
#include<vector>
#define IOS std::ios::sync_with_stdio(0)
using namespace std;
 const int N =1e5+5,mod=1e9+7;
 #define int long long
 bool ch[30][30];
 int n,m;
 char s[N] ;
 struct matrix {
    int a[30+2][30+2];
  };
  matrix m1;
   
  void init_(matrix &x){
    int i,j;
    for(i=1;i<=26;i++)
     for(j=1;j<=26;j++) {
         x.a[i][j]=0;
         if(i==j) x.a[i][j]=1;
     }
  }
  matrix mul(matrix &x,matrix &y){
    int i,j,k;
    matrix z;
     
    for(i=1;i<=26;i++)
     for(j=1;j<=26;j++){
        z.a[i][j]=0;
        for(k=1;k<=26;k++)
         z.a[i][j]+=x.a[i][k]*y.a[k][j]%mod, z.a[i][j]%=mod;
     }
     
    return z;
 }
   matrix ksm(matrix &x,int k){
    matrix tmp=x, ans;
    init_(ans);
     
    while(k){
        if(k&1) ans=mul(ans,tmp);
         
        tmp=mul(tmp,tmp);
        k/=2;
    }
    return ans;
 }
  
 signed main(){
    cin>>n;
    matrix Z;
    int i,j;
    for(i=1;i<=26;i++) 
    	for(j=1;j<=26;j++) Z.a[i][j]=1;
    cin>>s+1; m=strlen(s+1);
    
    for(i=2;i<=m;i++){
        char c1=s[i]-'a'+1,c2=s[i-1]-'a'+1 ;
        Z.a[c2][c1]=0;
    }
    matrix F;
    for(j=1;j<=26;j++) F.a[1][j] =1;
     
    matrix tmp =ksm(Z,n-1) ;
    F=mul(F,tmp);
    
    int ans=0;
    for(i=1;i<=26;i++)
        ans+= F.a[1][i],ans%=mod;
    cout<<ans<<endl;
 }