Hard data structure and algorithm practice puzzle.
Given a router count, a list of links as [a, b, cost] and two endpoints, return the cheapest total cost between them, or -1 when unreachable. Links do not all cost the same once you weight them by bandwidth, so hop counting gives the wrong route and breadth-first search stops being correct. Take the cheapest unfinished router each round and relax its neighbours: that is Dijkstra, and it is what OSPF runs on every router in the area.
Solve it three ways: snap the jigsaw pieces together, fill in the blanks, or type it from scratch against the tests. This one is free.
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