矩阵快速幂

求矩阵的 k 次幂

 

#define int long long
 #define N 3
  int mod =1e9+7;
 
 
  struct matrix {
      int a[N+2][N+2];
  };
  int n;
  
  void init_(matrix &x){
      memset(x.a,0,sizeof x.a) ;
      for(int i=0;i<N;i++){
            x.a[i][i]=1;
       }
  }
  matrix mul(matrix &x,matrix &y){
     matrix z;
     
     for(int i=0;i<N;i++)
      for(int j=0;j<N;j++){
          z.a[i][j]=0;
          for(int k=0;k<N;k++)
           z.a[i][j]+=x.a[i][k]*y.a[k][j], z.a[i][j]%=mod;
      }
     
     return z;
 }
   matrix ksm(matrix &x,int k){
     matrix tmp=x, ans;
     init_(ans);
     
     for(;k;k/=2,tmp=mul(tmp,tmp))if(k&1) ans=mul(ans,tmp);
     
     return ans;
 }