Hard data structure and algorithm practice puzzle.
Given a directed graph as an adjacency list, return an order where every node comes before the nodes it points to. If there's a cycle, return []. Use Kahn's algorithm: count each node's incoming edges, start a queue with the nodes at 0, and every time you output a node, lower its neighbors' counts. If you output fewer nodes than the graph has, there's a cycle.
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 Order Blast Radius, a pipeline you can run and download. See every Software Engineer project.
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