MediumCoding

Course Schedule

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1Give yourself 5 minutes

2Answer out loud, not in your head

3Then compare with the answer below

Stuck? Show a way to structure it+

01Build adjacency lists and indegrees.
02Queue every course with indegree zero.
03Remove outgoing edges while counting completed courses.
04A complete count means no directed cycle.

Reference answer

Model each prerequisite as a directed edge from prerequisite to dependent course. Kahn's algorithm starts with all zero-indegree courses, repeatedly takes one, decrements its neighbors' indegrees, and enqueues newly free courses. If the processed count equals the number of courses, a topological order exists and all courses can be finished; otherwise a cycle remains. Time is O(V+E) and space is O(V+E). DFS with visiting/visited states is also valid, but do not treat merely revisiting a finished node as a cycle.

Then expect these follow-ups

How would you return one valid course order?
Tests: topological ordering
How would you identify a concrete cycle?
Tests: DFS parent tracking

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