topological sorting in python

python/43_topological_sorting/topological_sorting.py

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+"""
+    Author: Wenru Dong
+"""
+
+from collections import deque
+from itertools import filterfalse
+
+class Graph:
+    def __init__(self, num_vertices: int):
+        self._num_vertices = num_vertices
+        self._adjacency = [[] for _ in range(num_vertices)]
+    
+    def add_edge(self, s: int, t: int) -> None:
+        self._adjacency[s].append(t)
+
+    def tsort_by_kahn(self) -> None:
+        in_degree = [0] * self._num_vertices
+        for v in range(self._num_vertices):
+            if len(self._adjacency[v]):
+                for neighbour in self._adjacency[v]:
+                    in_degree[neighbour] += 1
+        q = deque(filterfalse(lambda x: in_degree[x], range(self._num_vertices)))
+        while q:
+            v = q.popleft()
+            print(f"{v} -> ", end="")
+            for neighbour in self._adjacency[v]:
+                in_degree[neighbour] -= 1
+                if not in_degree[neighbour]:
+                    q.append(neighbour)
+        print("\b\b\b   ")
+
+    def tsort_by_dfs(self) -> None:
+        inverse_adjacency = [[] for _ in range(self._num_vertices)]
+        for v in range(self._num_vertices):
+            if len(self._adjacency[v]):
+                for neighbour in self._adjacency[v]:
+                    inverse_adjacency[neighbour].append(v)
+        visited = [False] * self._num_vertices
+
+        def dfs(vertex: int) -> None:
+            if len(inverse_adjacency[vertex]):
+                for v in inverse_adjacency[vertex]:
+                    if not visited[v]:
+                        visited[v] = True
+                        dfs(v)
+            print(f"{vertex} -> ", end="")
+        
+        for v in range(self._num_vertices):
+            if not visited[v]:
+                visited[v] = True
+                dfs(v)
+        
+        print("\b\b\b   ")
+
+
+if __name__ == "__main__":
+
+    dag = Graph(4)
+    dag.add_edge(1, 0)
+    dag.add_edge(2, 1)
+    dag.add_edge(1, 3)
+    dag.tsort_by_kahn()
+    dag.tsort_by_dfs()