Added Kahn's Algorithm in python

Author: cricketer94

Graph/TopoSort/Python/Kahn_Algorithm.py

@@ -0,0 +1,88 @@
+
+from collections import defaultdict
+ 
+#Class to represent a graph
+class Graph:
+    def __init__(self,vertices):
+        self.graph = defaultdict(list) #dictionary containing adjacency List
+        self.V = vertices #No. of vertices
+ 
+    # function to add an edge to graph
+    def addEdge(self,u,v):
+        self.graph[u].append(v)
+ 
+    def topological_sort(self):
+        
+        #initialise in_degrees
+	in_degree = [0 for i in range(self.V)]
+        
+        #calculate in_degrees of all vertices
+	for i in self.graph:
+	    for j in self.graph[i]:
+		in_degree[j]+=1
+        
+        #queue to keep track of vertices with 0 in_degree. These will be first in our topological ordering
+        queue = []
+        for i in range(self.V):
+            if(in_degree[i] == 0):
+		    queue.append(i)
+  
+
+        #list for our topological ordering
+        top_order = []       
+
+        # keep track of visited vertices to ensure there is no cycle
+        cnt = 0
+
+
+        #run loop until all vertices are added
+        while queue:
+            u = queue.pop(0)
+            top_order.append(u)
+
+            #remove edges outgoing from u
+            for vertex in self.graph[u]:
+                in_degree[vertex] -= 1
+
+                #new vertices which are "roots" get added to the queue
+                if in_degree[vertex] == 0:
+		            queue.append(vertex)
+	        cnt += 1
+
+        if cnt != self.V:
+            print "No topolocial ordering exists."
+
+        else:
+            print top_order
+
+        
+
+       
+	
+#Normal case
+g= Graph(6)
+g.addEdge(5, 2);
+g.addEdge(5, 0);
+g.addEdge(4, 0);
+g.addEdge(4, 1);
+g.addEdge(2, 3);
+g.addEdge(3, 1);
+ 
+print "Following is a Topological Sort of the given graph"
+g.topological_sort()
+
+
+
+# Cyclic graph
+g2= Graph(6)
+g2.addEdge(5, 2);
+g2.addEdge(2, 5);
+g2.addEdge(4, 0);
+g2.addEdge(4, 1);
+g2.addEdge(2, 3);
+g2.addEdge(3, 1);
+g2.addEdge(5, 0);
+ 
+print "Following is a Topological Sort of the given graph"
+g2.topological_sort()
+