course-schedule dfs py3

0207_course-schedule/python3/dfs.py

@@ -0,0 +1,38 @@
+from collections import defaultdict
+
+class Solution:
+    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
+        graph = defaultdict(list)
+        
+        for course, pre in prerequisites:
+            graph[course].append(pre)
+        
+        cycles = set()
+        visited = set()
+        
+        def dfs(course):
+            if course in cycles:
+                return False
+            
+            if course in visited:
+                return True
+            
+            visited.add(course)
+            cycles.add(course)
+            
+            for dep in graph[course]:
+                if not dfs(dep):
+                    return False
+                
+            cycles.remove(course)
+            
+            return True
+        
+        for course in range(numCourses):
+            cycles = set()
+            
+            if not dfs(course):
+                return False
+        
+        return True
+