Graphs · stretch

An order to take courses

needs maps each course to the courses it requires first. Write course_order(needs) returning a list of every course such that each comes after everything it needs — or None if that is impossible because the requirements loop.

When several courses are ready at once, take them in alphabetical order, so there is one right answer.

right answers
arguments left as they should be

def course_order(needs):
    return sorted(needs)

Run adds print(course_order({"algebra": [], "calculus": ["algebra"], "physics": ["calculus"]})) after your code, to try it.

Stuck on the idea rather than the code? The Groups and orderings lesson walks through it.