Hard data structure and algorithm practice puzzle.
Given a weighted graph, where each node lists [neighbor, cost] pairs, return the cheapest total cost from src to target, or -1 if you can't get there. Use Dijkstra's algorithm. Always pick the cheapest node you haven't finished yet, mark it finished, and update its neighbors. It only works because no cost is negative.
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.
Every line of the solution comes with a note explaining why it is there, and a step by step replay shows the code running. At level 3, where you type it yourself, you can ask for an AI mock interview on this problem that ends with a hiring manager's scorecard.
Then grow it into a real project with the Software Engineer button: this puzzle becomes the Trip Reachability Map, a pipeline you can run and download. See every Software Engineer project.
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