Course Schedule 25 XP Medium

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Given numCourses and prerequisites [a,b] meaning 'must take b before a', determine if you can finish all courses. This is cycle detection in a directed graph.

numCourses = 4
prereqs = [[1,0],[2,0],[3,1],[3,2]]
# 0 -> 1 -> 3
# 0 -> 2 -> 3
# No cycle, can finish = True

Use DFS with 3 states: unvisited, in-progress (currently in DFS path), visited (done). If we revisit an in-progress node, there's a cycle.

💡 Pro tip: Understand this problem deeply — don't just memorize the code. Try explaining the approach out loud as if teaching a friend. If you can explain it simply, you truly understand it!

📋 Instructions

Return True if all courses can be finished, False if there's a cycle.

DFS with states: 0=unvisited, 1=in-progress, 2=done. If you hit a node in state 1, it's a cycle.

🧪 Test Cases

Input

canFinish(4, [[1,0],[2,0],[3,1],[3,2]])

Expected

True

Boolean check

Input

canFinish(2, [[1,0],[0,1]])

Expected

False

Boolean check

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main.py

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Output

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