add graph topological sort

graph/topological_sort.py

@@ -0,0 +1,46 @@
+""" Topological Sort
+
+Topological sorting for Directed Acyclic Graph (DAG) is a linear ordering of
+vertices such that for every directed edge uv, vertex u comes before v in
+the ordering. Topological Sorting for a graph is not possible if the graph is
+not a DAG.
+"""
+
+def topological_sort(graph):
+    """ topological sort python implementation """
+    stack = []
+    visited = set()
+
+    def topological_sort_util(vertex):
+        """ modified depth-first search recursive algorithm """
+        visited.add(vertex)
+        for node in graph[vertex]:
+            if node not in visited:
+                topological_sort_util(node)
+        stack.append(vertex)
+
+
+    for vertex in list(graph):
+        if vertex not in visited:
+            topological_sort_util(vertex)
+
+    stack.reverse()
+    return stack
+
+
+def main():
+    """ operational function """
+    graph = {
+        0: [1, 2, 5],
+        1: [4],
+        2: [],
+        3: [2, 4, 5, 6],
+        4: [],
+        5: [2],
+        6: [0, 4]
+    }
+    print(topological_sort(graph))
+
+
+if __name__ == "__main__":
+    main()