Python/graphs/breadth_first_search.py
Casper Rysgaard e6cf13cc03
Update queue implementation (#5388)
* Update queue implementation

Popping the first element of a list takes O(n) time.
Using a cyclic queue takes O(1) time.

* Add queue changes from extra files

* Update indentation

* Add empty line between imports

* Fix lines

* Apply suggestions from code review

Co-authored-by: John Law <johnlaw.po@gmail.com>

Co-authored-by: John Law <johnlaw.po@gmail.com>
2021-10-30 19:06:25 +08:00

93 lines
2.3 KiB
Python

#!/usr/bin/python
""" Author: OMKAR PATHAK """
from __future__ import annotations
from queue import Queue
class Graph:
def __init__(self) -> None:
self.vertices: dict[int, list[int]] = {}
def print_graph(self) -> None:
"""
prints adjacency list representation of graaph
>>> g = Graph()
>>> g.print_graph()
>>> g.add_edge(0, 1)
>>> g.print_graph()
0 : 1
"""
for i in self.vertices:
print(i, " : ", " -> ".join([str(j) for j in self.vertices[i]]))
def add_edge(self, from_vertex: int, to_vertex: int) -> None:
"""
adding the edge between two vertices
>>> g = Graph()
>>> g.print_graph()
>>> g.add_edge(0, 1)
>>> g.print_graph()
0 : 1
"""
if from_vertex in self.vertices:
self.vertices[from_vertex].append(to_vertex)
else:
self.vertices[from_vertex] = [to_vertex]
def bfs(self, start_vertex: int) -> set[int]:
"""
>>> g = Graph()
>>> g.add_edge(0, 1)
>>> g.add_edge(0, 1)
>>> g.add_edge(0, 2)
>>> g.add_edge(1, 2)
>>> g.add_edge(2, 0)
>>> g.add_edge(2, 3)
>>> g.add_edge(3, 3)
>>> sorted(g.bfs(2))
[0, 1, 2, 3]
"""
# initialize set for storing already visited vertices
visited = set()
# create a first in first out queue to store all the vertices for BFS
queue = Queue()
# mark the source node as visited and enqueue it
visited.add(start_vertex)
queue.put(start_vertex)
while not queue.empty():
vertex = queue.get()
# loop through all adjacent vertex and enqueue it if not yet visited
for adjacent_vertex in self.vertices[vertex]:
if adjacent_vertex not in visited:
queue.put(adjacent_vertex)
visited.add(adjacent_vertex)
return visited
if __name__ == "__main__":
from doctest import testmod
testmod(verbose=True)
g = Graph()
g.add_edge(0, 1)
g.add_edge(0, 2)
g.add_edge(1, 2)
g.add_edge(2, 0)
g.add_edge(2, 3)
g.add_edge(3, 3)
g.print_graph()
# 0 : 1 -> 2
# 1 : 2
# 2 : 0 -> 3
# 3 : 3
assert sorted(g.bfs(2)) == [0, 1, 2, 3]