solve: courseSchedule

Author: GaggleHelena

course-schedule/yolophg.py

@@ -0,0 +1,43 @@
+# Time Complexity: O(V + E) - visit each course once and process all edges
+# Space Complexity: O(V + E) - storing the graph + recursion stack (worst case)
+
+class Solution:
+    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
+
+        # Build the adjacency list (prereq map)
+        preMap = {i: [] for i in range(numCourses)}
+        for crs, pre in prerequisites:
+            preMap[crs].append(pre)
+        
+        # tracks courses currently being checked (for cycle detection)
+        visited = set()  
+
+        def dfs(crs):
+            # found a cycle
+            if crs in visited:  
+                return False
+            # no more prereqs left
+            if preMap[crs] == []: 
+                return True
+            
+            # mark as visiting
+            visited.add(crs)  
+            
+            # check all prereqs for this course
+            for pre in preMap[crs]:
+                if not dfs(pre):
+                    # cycle detected, return False
+                    return False  
+            
+            # done checking, remove from visited
+            visited.remove(crs)  
+            # mark as completed
+            preMap[crs] = []  
+            return True
+
+        # run dfs on every course
+        for crs in range(numCourses):
+            if not dfs(crs):
+                return False
+        
+        return True