Language: C++

#include <bits/stdc++.h>

#define M_PI       3.14159265358979323846   // pi

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<ll> VI;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> t3;
typedef tuple<ll, ll, ll, ll> t4;

#define rep(a,n) for(ll a = 0;a < n;a++)
#define repi(a,b,n) for(ll a = b;a < n;a++)

#include <bits/stdc++.h>
using namespace std;

template<typename T>
void cmpmax(T* reference, T value) {
	*reference = max(*reference, value);
}

template<typename T>
void cmpmin(T* reference, T value) {
	*reference = min(*reference, value);
}

const ll mod = 1000000007;

static const ll INF = 1e15;

#pragma once

template< class T >
struct Matrix {
	vector< vector< T > > A;

	Matrix() {}

	Matrix(size_t n, size_t m) : A(n, vector< T >(m, 0)) {}

	Matrix(size_t n) : A(n, vector< T >(n, 0)) {};

	size_t height() const {
		return (A.size());
	}

	size_t width() const {
		return (A[0].size());
	}

	inline const vector< T >& operator[](int k) const {
		return (A.at(k));
	}

	inline vector< T >& operator[](int k) {
		return (A.at(k));
	}

	static Matrix Identity(size_t n) {
		Matrix mat(n);
		for (int i = 0; i < n; i++) mat[i][i] = 1;
		return (mat);
	}

	Matrix& operator+=(const Matrix& B) {
		size_t n = height(), m = width();
		assert(n == B.height() && m == B.width());
		for (int i = 0; i < n; i++)
			for (int j = 0; j < m; j++)
				(*this)[i][j] += B[i][j];
		return (*this);
	}

	Matrix& operator-=(const Matrix& B) {
		size_t n = height(), m = width();
		for (int i = 0; i < n; i++)
			for (int j = 0; j < m; j++)
				(*this)[i][j] -= B[i][j];
		return (*this);
	}

	Matrix& operator*=(const Matrix& B) {
		size_t n = height(), m = B.width(), p = width();
		vector< vector< T > > C(n, vector< T >(m, 0));
		for (int i = 0; i < n; i++)
			for (int j = 0; j < m; j++)
				for (int k = 0; k < p; k++)
				{
					C[i][j] = (C[i][j] + (*this)[i][k] * B[k][j]);
					C[i][j %= mod;
				}
		A.swap(C);
		return (*this);
	}

	Matrix& operator^=(long long k) {
		Matrix B = Matrix::Identity(height());
		while (k > 0) {
			if (k & 1) B *= *this;
			*this *= *this;
			k >>= 1LL;
		}
		A.swap(B.A);
		return (*this);
	}

	Matrix& apply(std::function<T(T)> operation) {
		size_t n = height(), m = width();
		for (int i = 0; i < n; i++)
			for (int j = 0; j < m; j++)
				(*this)[i][j] = operation((*this)[i][j]);
		return (*this);
	}

	Matrix& pow(long long k, long long mod) {
		Matrix B = Matrix::Identity(height());
		while (k > 0) {
			if (k & 1) {
				B *= *this;
				B.apply([&](long long v) {return v % mod; });
			}
			*this *= *this;
			apply([&](long long v) {return v % mod; });
			k >>= 1LL;
		}
		A.swap(B.A);
		return (*this);
	}

	vector<T> product(const vector<T>& right) {
		assert(width() == right.size());
		vector<T> left(height());
		for (int y = 0; y < height(); y++) {
			T sum = 0;
			for (int x = 0; x < width(); x++) {
				sum += this[y][x] * right[x];
			}
			left[y] = sum;
		}
		return left;
	}

	Matrix operator+(const Matrix& B) const {
		return (Matrix(*this) += B);
	}

	Matrix operator-(const Matrix& B) const {
		return (Matrix(*this) -= B);
	}

	Matrix operator*(const Matrix& B) const {
		return (Matrix(*this) *= B);
	}

	Matrix operator^(const long long k) const {
		return (Matrix(*this) ^= k);
	}

	friend ostream& operator<<(ostream& os, Matrix& p) {
		size_t n = p.height(), m = p.width();
		for (int i = 0; i < n; i++) {
			os << "[";
			for (int j = 0; j < m; j++) {
				os << p[i][j] << (j + 1 == m ? "]\n" : ",");
			}
		}
		return (os);
	}


	T determinant() {
		Matrix B(*this);
		assert(width() == height());
		T ret = 1;
		for (int i = 0; i < width(); i++) {
			int idx = -1;
			for (int j = i; j < width(); j++) {
				if (B[j][i] != 0) idx = j;
			}
			if (idx == -1) return (0);
			if (i != idx) {
				ret *= -1;
				swap(B[i], B[idx]);
			}
			ret *= B[i][i];
			T vv = B[i][i];
			for (int j = 0; j < width(); j++) {
				B[i][j] /= vv;
			}
			for (int j = i + 1; j < width(); j++) {
				T a = B[j][i];
				for (int k = 0; k < width(); k++) {
					B[j][k] -= B[i][k] * a;
				}
			}
		}
		return (ret);
	}
};

int main() {
	ll n, k;
	cin >> n >> k;
	Matrix<ll> mat(n, n);
	rep(y, n) {
		rep(x, n) {
			cin >> mat[y][x];
		}
	}
	mat.pow(k, mod);
	ll sum = 0;
	rep(y, n) {
		rep(x, n) {
			sum += mat[y][x];
			sum %= mod;
		}
	}
	cout << sum << endl;
	return 0;
}
	


Compiler output:
a.cc:33:9: warning: #pragma once in main file
   33 | #pragma once
      |         ^~~~
a.cc: In member function 'Matrix<T>& Matrix<T>::operator*=(const Matrix<T>&)':
a.cc:92:54: error: expected ']' before ';' token
   92 |                                         C[i][j %= mod;
      |                                                      ^
a.cc: In instantiation of 'Matrix<T>& Matrix<T>::operator*=(const Matrix<T>&) [with T = long long int]':
a.cc:121:7:   required from 'Matrix<T>& Matrix<T>::pow(long long int, long long int) [with T = long long int]'
  121 |                                 B *= *this;
      |                                 ~~^~~~~~~~
a.cc:212:9:   required from here
  212 |         mat.pow(k, mod);
      |         ~~~~~~~^~~~~~~~
a.cc:92:45: warning: ignoring return value of 'std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::operator[](size_type) [with _Tp = long long int; _Alloc = std::allocator<long long int>; reference = long long int&; size_type = long unsigned int]', declared with attribute 'nodiscard' [-Wunused-result]
   92 |                                         C[i][j %= mod;
      |                                         ~~~~^
In file included from /usr/include/c++/14/vector:66,
                 from /usr/include/c++/14/functional:64,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:53,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_vector.h:1128:7: note: declared here
 1128 |       operator[](size_type __n) _GLIBCXX_NOEXCEPT
      |       ^~~~~~~~