They threw in a simplifying constraint: each course has at most one direct prerequisite, so the graph is actually a forest rather than a general DAG.
Model the courses and prerequisites as a directed graph and use topological sorting to find a valid order. Apply Kahn's algorithm (BFS with in-degree tracking) or DFS with cycle detection, and clearly explain how you handle cycles by returning an empty order.
Pro tip: Mention that you would detect cycles and return an empty array if no valid order exists, and discuss the trade-offs between Kahn's algorithm and DFS-based topological sort in terms of readability and performance.
Confirm that the input is a list of prerequisite pairs where [a, b] means b must be taken before a. Ask about possible cycles, duplicate edges, and whether all courses need to be included.
Decide between Kahn's algorithm (BFS with in-degree) and DFS with cycle detection. Explain your choice based on simplicity and efficiency.
Create an adjacency list for the directed graph and an array to track in-degrees for each node.
Use a queue to process nodes with in-degree 0, appending them to the result and decrementing in-degrees of their neighbors. For DFS, use recursion with temporary and permanent marks to detect cycles.
Check if the result contains all n courses. If not, a cycle exists; return an empty array. Otherwise, return the order.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.