Create kosaraju's algorithm.py

Python/kosaraju's algorithm.py

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+# Python implementation of Kosaraju's algorithm to print all SCCs 
+
+from collections import defaultdict 
+
+#This class represents a directed graph using adjacency list representation 
+class Graph: 
+
+	def __init__(self,vertices): 
+		self.V= vertices #No. of vertices 
+		self.graph = defaultdict(list) # default dictionary to store graph 
+
+	# function to add an edge to graph 
+	def addEdge(self,u,v): 
+		self.graph[u].append(v) 
+
+	# A function used by DFS 
+	def DFSUtil(self,v,visited): 
+		# Mark the current node as visited and print it 
+		visited[v]= True
+		print v, 
+		#Recur for all the vertices adjacent to this vertex 
+		for i in self.graph[v]: 
+			if visited[i]==False: 
+				self.DFSUtil(i,visited) 
+
+
+	def fillOrder(self,v,visited, stack): 
+		# Mark the current node as visited 
+		visited[v]= True
+		#Recur for all the vertices adjacent to this vertex 
+		for i in self.graph[v]: 
+			if visited[i]==False: 
+				self.fillOrder(i, visited, stack) 
+		stack = stack.append(v) 
+
+
+	# Function that returns reverse (or transpose) of this graph 
+	def getTranspose(self): 
+		g = Graph(self.V) 
+
+		# Recur for all the vertices adjacent to this vertex 
+		for i in self.graph: 
+			for j in self.graph[i]: 
+				g.addEdge(j,i) 
+		return g 
+
+
+
+	# The main function that finds and prints all strongly 
+	# connected components 
+	def printSCCs(self): 
+
+		stack = [] 
+		# Mark all the vertices as not visited (For first DFS) 
+		visited =[False]*(self.V) 
+		# Fill vertices in stack according to their finishing 
+		# times 
+		for i in range(self.V): 
+			if visited[i]==False: 
+				self.fillOrder(i, visited, stack) 
+
+		# Create a reversed graph 
+		gr = self.getTranspose() 
+
+		# Mark all the vertices as not visited (For second DFS) 
+		visited =[False]*(self.V) 
+
+		# Now process all vertices in order defined by Stack 
+		while stack: 
+			i = stack.pop() 
+			if visited[i]==False: 
+				gr.DFSUtil(i, visited) 
+				print"" 
+
+# Create a graph given in the above diagram 
+g = Graph(5) 
+g.addEdge(1, 0) 
+g.addEdge(0, 2)
+g.addEdge(0, 3) 
+g.addEdge(3, 4) 
+
+
+print ("Following are strongly connected components " +
+						"in given graph") 
+g.printSCCs()