CoursesTrees & GraphsShortest Paths
Shortest Path with Alternating Colors
The state-augmentation lesson: the BFS state is (node, last edge color), not the node — one node can be visited twice, once per arriving color.
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.
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.