class ClassRegistrationSystem:
    """
    This is a class as a class registration system, allowing to register students, register them for classes, retrieve students by major, get a list of all majors, and determine the most popular class within a specific major.
    """

    def __init__(self):
        self.students = []
        self.students_registration_classes = {}

    def register_student(self, student):
        """
        register a student to the system, add the student to the students list, if the student is already registered, return 0, else return 1
        """

        if student in self.students:
            return 0
        else:
            self.students.append(student)
            return 1

    def register_class(self, student_name, class_name):
        """
        register a class to the student.
        :param student_name: str
        :param class_name: str
        :return a list of class names that the student has registered
        >>> registration_system = ClassRegistrationSystem()
        >>> registration_system.register_class(student_name="John", class_name="CS101")
        >>> registration_system.register_class(student_name="John", class_name="CS102")
        ["CS101", "CS102"]

        if student_name in self.students_registration_classes:
            self.students_registration_classes[student_name].append(class_name)
        else:
            self.students_registration_classes[student_name] = [class_name]
        return self.students_registration_classes[student_name]

    def get_students_by_major(self, major):
        """
        get all students in the major
        :param major: str
        :return a list of student name
        >>> registration_system = ClassRegistrationSystem()
        >>> student1 = {"name": "John", "major": "Computer Science"}
        >>> registration_system.register_student(student1)
        >>> registration_system.get_students_by_major("Computer Science")
        ["John"]
        """

        student_list = []
        for student in self.students:
            if student["major"] == major:
                student_list.append(student["name"])
        return student_list

    def get_all_major(self):
        """
        get all majors in the system
        :return a list of majors
        >>> registration_system = ClassRegistrationSystem()
        >>> registration_system.students = [{"name": "John", "major": "Computer Science"}],
        >>> registration_system.get_all_major(student1)
        ["Computer Science"]
        """

        major_list = []
        for student in self.students:
            if student["major"] not in major_list:
                major_list.append(student["major"])
        return major_list

    def get_most_popular_class_in_major(self, major):
        class_list = []
        for student in self.students:
            if student["major"] == major:
                class_list += self.students_registration_classes[student["name"]]
        most_popular_class = max(set(class_list), key=class_list.count)
        return most_popular_class

Method:
def get_most_popular_class_in_major(self, major):
        class_list = []
        for student in self.students:
            if student["major"] == major:
                class_list += self.students_registration_classes[student["name"]]
        most_popular_class = max(set(class_list), key=class_list.count)
        return most_popular_class

Summary: get the class with the highest enrollment in the major.