Merge pull request #177 from jerryderry/bfs-dfs-python

bfs and dfs of graphs in python

python/31_bfs_dfs/bfs_dfs.py

@@ -0,0 +1,88 @@
+"""
+    Breadth-first search and depth-first search.
+
+    Author: Wenru Dong
+"""
+
+from typing import List, Optional, Generator, IO
+from collections import deque
+
+class Graph:
+    """Undirected 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)
+        self._adjacency[t].append(s)
+
+    def _generate_path(self, s: int, t: int, prev: List[Optional[int]]) -> Generator[str, None, None]:
+        if prev[t] or s != t:
+            yield from self._generate_path(s, prev[t], prev)
+        yield str(t)
+
+    def bfs(self, s: int, t: int) -> IO[str]:
+        """Print out the path from Vertex s to Vertex t
+        using bfs.
+        """
+        if s == t: return
+
+        visited = [False] * self._num_vertices
+        visited[s] = True
+        q = deque()
+        q.append(s)
+        prev = [None] * self._num_vertices
+
+        while q:
+            v = q.popleft()
+            for neighbour in self._adjacency[v]:
+                if not visited[neighbour]:
+                    prev[neighbour] = v
+                    if neighbour == t:
+                        print("->".join(self._generate_path(s, t, prev)))
+                        return
+                    visited[neighbour] = True
+                    q.append(neighbour)
+
+    def dfs(self, s: int, t: int) -> IO[str]:
+        """Print out a path from Vertex s to Vertex t
+        using dfs.
+        """
+        found = False
+        visited = [False] * self._num_vertices
+        prev = [None] * self._num_vertices
+
+        def _dfs(from_vertex: int) -> None:
+            nonlocal found
+            if found: return
+            visited[from_vertex] = True
+            if from_vertex == t:
+                found = True
+                return
+            for neighbour in self._adjacency[from_vertex]:
+                if not visited[neighbour]:
+                    prev[neighbour] = from_vertex
+                    _dfs(neighbour)
+        
+        _dfs(s)
+        print("->".join(self._generate_path(s, t, prev)))
+
+
+if __name__ == "__main__":
+    
+    graph = Graph(8)
+
+    graph.add_edge(0, 1)
+    graph.add_edge(0, 3)
+    graph.add_edge(1, 2)
+    graph.add_edge(1, 4)
+    graph.add_edge(2, 5)
+    graph.add_edge(3, 4)
+    graph.add_edge(4, 5)
+    graph.add_edge(4, 6)
+    graph.add_edge(5, 7)
+    graph.add_edge(6, 7)
+
+    graph.bfs(0, 7)
+    graph.dfs(0, 7)