Detect Cycle in Directed Graph

Author: OmkarPathak

Graph/P03_DetectCycleInDirectedGraph.py

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+# Author: OMKAR PATHAK
+
+# Time Complexity: O(|V| + |E|)
+class Graph():
+    def __init__(self):
+        self.vertex = {}
+
+    # for printing the Graph vertexes
+    def printGraph(self):
+        for i in self.vertex.keys():
+            print(i,' -> ', ' -> '.join([str(j) for j in self.vertex[i]]))
+
+    # for adding the edge beween two vertexes
+    def addEdge(self, fromVertex, toVertex):
+        # check if vertex is already present,
+        if fromVertex in self.vertex.keys():
+            self.vertex[fromVertex].append(toVertex)
+        else:
+            # else make a new vertex
+            self.vertex[fromVertex] = [toVertex]
+
+    # This function will return True if graph is cyclic else return False
+    def checkCyclic(self):
+        visited = [False] * len(self.vertex)
+        stack = [False] * len(self.vertex)
+        for vertex in range(len(self.vertex)):
+            if visited[vertex] == False:
+                if self.checkCyclicRec(visited, stack, vertex) == True:
+                    return True
+        return False
+
+    # Recursive function for finding the cycle
+    def checkCyclicRec(self, visited, stack, vertex):
+        # Mark the current node in visited and also add it to the stack
+        visited[vertex] = True
+        stack[vertex] = True
+
+        # mark all adjacent nodes of the current node
+        for adjacentNode in self.vertex[vertex]:
+            if visited[adjacentNode] == False:
+                if self.checkCyclicRec(visited, stack, adjacentNode) == True:
+                    return True
+            elif stack[adjacentNode] == True:
+                return True
+
+        # The node needs to be poped from
+        # recursion stack before function ends
+        stack[vertex] = False
+        return False
+
+if __name__ == '__main__':
+    graph = Graph()
+    graph.addEdge(0, 1)
+    graph.addEdge(0, 2)
+    graph.addEdge(1, 2)
+    graph.addEdge(2, 0)
+    graph.addEdge(2, 3)
+    graph.addEdge(3, 3)
+
+    graph.printGraph()
+
+    if graph.checkCyclic() == 1:
+        print ("Graph has a cycle")
+    else:
+        print ("Graph has no cycle")