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Group DAG tasks into parallel waves

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Group DAG tasks into parallel waves

Given a DAG as deps (task → list of prerequisites), group tasks into execution waves: wave 0 is everything with no prerequisites; each later wave is the tasks whose prerequisites have *all* completed in earlier waves. Tasks within a wave can run in parallel.

Return a list of waves, each a sorted list of task names. Include tasks that appear only as prerequisites. If the graph has a cycle, return None.

Python 3.12, standard library only — no pip installs.

solution.py

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