For a given route, it decides which stations to stop at and how much fuel to buy at each one, minimizing total fuel cost while guaranteeing the truck never runs dry. It's built on the classic "gas station problem": at each candidate stop, if the current stop is cheaper than the next one, fill up completely; if the next stop is cheaper, fill only enough to reach it; at the final stop before the destination, fill only enough to arrive with the desired reserve.