CoursesBacktracking & GreedyCombinations
Choose k of n
Build every size-k selection from 1..n with the start cursor, and abandon any branch that no longer has enough numbers left to fill its empty slots.
AlgoFox Pro
Backtracking & Greedy is part of AlgoFox Pro
You can read the whole course outline here. The exercises are in the app, along with your streak, your mistake review, and the progress that carries across your devices.
Pro starts with a 3 day free trial. The app store decides who is eligible for an introductory offer, and a lapsed subscription does not get another one.
Already subscribed in the app? Sign in with the same account and Pro works here too. Quick Play and Battle are free on the web, and so is the first course in every subject.