add detect cycle in DAG python

Author: kumailn

Python/Cycle_In_DAG.py

@@ -0,0 +1,40 @@
+def hasCycle(edges, numEdges):
+    #Initialize a graph that stores the neighbor of each node
+    graph = {i:[] for i in list(range(numEdges))}
+    #Populate the graph so that the 'children' of each node are stored with the parent as the key
+    for x, y in edges: graph[x] += [y]
+    #Create three sets, one to store all unvisited nodes, one to store nodes curently visiting, one to store completely visited ones
+    unvisited = list(range(numEdges))
+    visiting = set()
+    completelyVisited = set()
+    
+    #While we still have nodes that are unvisited
+    while unvisited:
+        #We graph a random node from the unvisited collection to run a DFS on
+        currentNode = unvisited[0]
+        #If a call to the depth first seach returns true then we know this graph has a cycle and can return true
+        if depthFirstSearch(currentNode, unvisited, visiting, completelyVisited, graph): return True
+    #If no call to the depth first search returns true then we can return False that there is no cycle in this graph
+    return False
+
+#Define our helper depth first search for a cycle
+def depthFirstSearch(currentNode, unvisited, visiting, completelyVisited, graph):
+        #Firstly, we need to move the current node we're at from unvisited and to visiting
+        unvisited.remove(currentNode)
+        visiting.add(currentNode)
+        #Now for each of this nodes adjacent neighbors we need to run this DFS again
+        for neighbor in graph[currentNode]:
+            #If that neighbor has already been completely visited, we can skip tis particular neighbor
+            if neighbor in completelyVisited: continue
+            #If this neighbor is already a node we're visiting then return true that this has a cycle! Because we've circled back to a parent node
+            if neighbor in visiting: return True
+            #If any its neighbors have a cycle return true as well
+            if depthFirstSearch(neighbor, unvisited, visiting, completelyVisited, graph): return True
+        #Finally remove this current node from visiting and add it to completely visited
+        visiting.remove(currentNode)
+        completelyVisited.add(currentNode)
+
+def main():
+    print(hasCycle([[1, 0], [0, 2]], 3))
+
+main()