I think levels 18 and 19 should be switched, since 18 is much harder than 19. Also, scoring should be based on moves instead of time. Otherwise it's a great puzzle game!
This is an interesting (and fun!) application of graph theory, specifically on finding Euler trails in graphs. I'll have to point this out to my former Graph Theory teacher. Thanks!
lvl18 and lvl19 have been swapped =)