How do you produce a valid course order given prerequisites, or detect that it is impossible?
PICTURE THIS: STACK VS QUEUE
Simple meaning
Kahn: queue all zero-indegree courses, append them to the order, and decrement neighbors.
WHY — Graphs instead of guessing?
Why interviewers care about Graphs:
question about Graphs.
trade-offs, and what you would actually do on a DSA project - not buzzwords.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1Kahn: queue all zero-indegree
courses, append them to the order, and decrement neighbors.
- 2If the order length
is less than n, a cycle exists.
- 3Time is O(V+E) and
space is O(V+E).
- 4DFS finishing times reverse
to a topo order if you abort on a gray back edge.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
Kahn: queue all zero-indegree courses, append them to the order, and decrement neighbors. If the order length is less than n, a cycle exists.