Oh Hi (https://play.google.com/store/apps/details?id=com.presto.ohh...) is an app I have that has these puzzles.
Simple rules and can get surprisingly difficult.
Simple rules and can get surprisingly difficult.
https://play.google.com/store/apps/details?id=com.q42.ohhi&h...
EDIT: No way to edit old posts, but the link I posted is obviously a rip-off of the Q42 app linked elsewhere.