AlgoFoxGet AlgoFox App

CoursesTrees & GraphsGrid Traversal

Surrounded Regions

Don't hunt the surrounded regions — flood from the border 'O's, mark the survivors safe, and flip everything else.

Complete codeDebugCode orderMultiple choiceTrace6 exercises, 7 min
AlgoFox Pro

Trees & Graphs 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.

Download AlgoFox on the App StoreGet AlgoFox on Google Play

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.