1def organize(cars, students):
 2    if len(students) > len(cars) * 4:
 3        return False
 4
 5    # represents the cars each student can be in initially
 6    students_cars = determine_cars(cars, students)
 7    # represents the friends of each student when both are placed in a comfortable car
 8    students_friends = determine_friends(cars, students_cars, students)
 9
10    passengers = {index: set() for index in range(len(cars))}
11    students_idx = [index for index in range(len(students))]
12
13    # backtracking
14    def find_accommodation(passengers, students_idx):
15        if not students_idx:
16            return True
17
18        current_student = students_idx[0]
19
20        for car, car_passengers in passengers.items():
21            try:
22                if all([comfy_in_car(students_cars, current_student, car) and 
23                        are_friends(students_friends, current_student, passenger)
24                        for passenger in car_passengers]):
25                    cars[car].add_student(students[current_student])
26                    passengers[car].add(current_student)
27
28                    if find_accommodation(passengers, students_idx[1:]):
29                        return True
30
31                    cars[car].remove_student(students[current_student])
32                    passengers[car].remove(current_student)
33            except EnvironmentError:
34                continue
35
36        return False
37
38    return find_accommodation(passengers, students_idx)
39
40
41def are_friends(student_friends, student1_idx, student2_idx):
42    return student2_idx in student_friends.get(student1_idx)
43
44
45def comfy_in_car(student_cars, student_idx, car_idx):
46    return car_idx in student_cars.get(student_idx)
47
48
49def determine_cars(cars, students):
50    comfy_cars = {index: set() for index in range(len(students))}
51
52    for student_idx in range(len(students)):
53        for car_idx in range(len(cars)):
54            try:
55                cars[car_idx].add_student(students[student_idx])
56                if students[student_idx].is_comfy():
57                    comfy_cars[student_idx].add(car_idx)
58                cars[car_idx].remove_student(students[student_idx])
59            except EnvironmentError:
60                continue
61    return comfy_cars
62
63
64def determine_friends(cars, comfy_cars, students):
65    friends = {index: set() for index in range(len(students))}
66
67    for student, st_cars in comfy_cars.items():
68        for other_student, other_cars in comfy_cars.items():
69            if student == other_student:
70                continue
71            common_cars = set.intersection(st_cars, other_cars)
72            if len(common_cars) == 0:
73                # they can never be together
74                continue
75            some_car = cars[common_cars.pop()]
76            try:
77                some_car.add_student(students[student])
78                some_car.add_student(students[other_student])
79
80                if students[student].is_comfy() and students[other_student].is_comfy():
81                    friends[student].add(other_student)
82                    friends[other_student].add(student)
83
84                some_car.remove_student(students[student])
85                some_car.remove_student(students[other_student])
86            except EnvironmentError:
87                continue
88    return friends