During relaxation (if you found a new, shorter path to a node), you don't just update the cost of getting there, but also save how you got there (i.e., from whi

During relaxation (if you found a new, shorter path to a node), you don't just update the cost of getting there, but also save how you got there (i.e., from which node). Then it's just a matter of retracing the steps from B to A!