题目：

题解：

const int MOD = 1000000007;struct Matrix {long mat[6][6];int row, col;
};struct Matrix multiply(struct Matrix a, struct Matrix b) {int rows = a.row, columns = b.col, temp = b.row;struct Matrix c;memset(c.mat, 0, sizeof(c.mat));c.row = rows, c.col = columns;for (int i = 0; i < rows; i++) {for (int j = 0; j < columns; j++) {for (int k = 0; k < temp; k++) {c.mat[i][j] += a.mat[i][k] * b.mat[k][j];c.mat[i][j] %= MOD;}}}return c;
}struct Matrix matricPow(struct Matrix mat, int n) {struct Matrix ret = {{{1, 0, 0, 0, 0, 0}}, 1, 6};while (n > 0) {if ((n & 1) == 1) {ret = multiply(ret, mat);}n >>= 1;mat = multiply(mat, mat);}return ret;
}int checkRecord(int n) {struct Matrix mat = {{{1, 1, 0, 1, 0, 0}, {1, 0, 1, 1, 0, 0}, {1, 0, 0, 1, 0, 0}, {0, 0, 0, 1, 1, 0}, {0, 0, 0, 1, 0, 1}, {0, 0, 0, 1, 0, 0}}, 6, 6};struct Matrix res = matricPow(mat, n);long sum = 0;for (int i = 0; i < res.col; i++) {sum += res.mat[0][i];}return (int)(sum % MOD);
}