Days Until a Harder Lesson
`difficulties` lists a difficulty rating for each day's lesson, in order. For each day, return how many days you'd have to wait until a STRICTLY harder lesson appears, or 0 if none ever does.
`difficulties` lists a difficulty rating for each day's lesson, in order. For each day, return how many days you'd have to wait until a STRICTLY harder lesson appears, or 0 if none ever does.
