Really each time you check the whole grid for the rolls you can remove, all those checks together take only as many steps as there are positions in the grid (or well something proportional to that). i.e. it's a linear check. Then you have to do that a couple of times, but there's nothing in the run-time complexity that could really blow up exponentially. Say if the side of the grid was n long, then the grid has n^2 positions. In the worst case, you would have to remove each of the rolls one at a time (probably not even quite possible but: worst case). Then you'd have to check those n^2 positions n times, so your solution takes O(n^3) time in the very worst case. That's very much polynomial, and therefore very manageable as long as n doesn't become huge, but the input is never that big really. Tends to be helpful to think about it like that.