Topological Sort

Author: OmkarPathak

Graph/P05_TopologicalSort.py

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+# Author: OMKAR PATHAK
+
+# Time Complexity: O(|V| + |E|)
+# One important point to remember is that topological sort can be applied only to acyclic graph.
+
+class Graph():
+    def __init__(self, count):
+        self.vertex = {}
+        self.count = count          # vertex count
+
+    # 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]
+            self.vertex[toVertex] = []
+
+    def topologicalSort(self):
+        visited = [False] * self.count           # Marking all vertices as not visited
+        stack = []                               # Stack for storing the vertex
+        for vertex in range(self.count):
+            # Call the recursive function only if not visited
+            if visited[vertex] == False:
+                self.topologicalSortRec(vertex, visited, stack)
+
+        print(' '.join([str(i) for i in stack]))
+        # print(stack)
+
+    # Recursive function for topological Sort
+    def topologicalSortRec(self, vertex, visited, stack):
+
+        # Mark the current node in visited
+        visited[vertex] = True
+
+        # mark all adjacent nodes of the current node
+        try:
+            for adjacentNode in self.vertex[vertex]:
+                if visited[adjacentNode] == False:
+                    self.topologicalSortRec(adjacentNode, visited, stack)
+        except KeyError:
+            return
+
+        # Push current vertex to stack which stores the result
+        stack.insert(0,vertex)
+
+if __name__ == '__main__':
+    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)
+    # g.printGraph()
+    g.topologicalSort()
+
+    # OUTPUT:
+    # 5 4 2 3 1 0