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; }