Update 0210-course-schedule-ii.py

11_Graphs/09_Course_Schedule_II/0210-course-schedule-ii.py

@@ -16,28 +16,34 @@
 
 class Solution:
     def findOrder(self, numCourses: int, prerequisites: List[List[int]]) -> List[int]:
-        graph = {i: [] for i in range(numCourses)}
-        in_degree = [0] * numCourses
-        order = []
-
-        # Construct the graph and count in-degrees
-        for course, prereq in prerequisites:
-            graph[prereq].append(course)
-            in_degree[course] += 1
-
-        # Initialize a queue with nodes having in-degree zero
-        queue = collections.deque(
-            [course for course, degree in enumerate(in_degree) if degree == 0]
-        )
-
-        # Perform topological sorting and update in-degrees
-        while queue:
-            node = queue.popleft()
-            order.append(node)
-            for neighbor in graph[node]:
-                in_degree[neighbor] -= 1
-                if in_degree[neighbor] == 0:
-                    queue.append(neighbor)
-
-        # If the order doesn't contain all courses, there's a cycle
-        return order if len(order) == numCourses else []
+        preReq = defaultdict(list)
+        #build graph
+        for course,p in prerequisites:
+            preReq[course].append(p)
+
+        res = []
+        visited, cycle = set(), set()
+
+        def dfs(course):
+            # terminating case
+            if course in cycle:
+                return False
+
+            if course in visited:
+                return True
+            else:
+                cycle.add(course)
+
+            for p in preReq[c]:
+                if not dfs(p):
+                    return False
+            cycle.remove(course). #after processing add to visited
+            visited.add(course)
+            res.append(course)
+            return True
+
+        for c in range(numCourses):
+            if dfs(c) == False:
+                return []
+
+        return res