karat student courses intersection

def solve(pairs: List[Tuple[str, str]]) -> Dict[Tuple[str, str], List[str]]:
    student_courses: Dict[str, Set[str]] = {}
    
    for student, course in pairs:
        if student not in student_courses:
            student_courses[student] = set()
        student_courses[student].add(course)
    
    ret = {}
    students = list(student_courses.keys())
    for i in range(len(students)):
        for j in range(i + 1, len(students)):
            pair = (students[i], students[j])
            student_i_courses = student_courses[students[i]]
            student_j_courses = student_courses[students[j]]
            intersect_courses = student_i_courses.intersection(student_j_courses)

            ret[pair] = list(intersect_courses)

    return ret

pairs = [
    ("s1", "a"),
    ("s2", "a"),
    ("s2", "b"),
    ("s3", "b"),
    ("s1", "c"),
    ("s1", "d"),
    ("s1", "e"),
    ("s3", "d"),
    ("s2", "c"),
]
print(solve(pairs))