mirror of
https://github.com/TheAlgorithms/Python.git
synced 2024-11-23 21:11:08 +00:00
c50f0c56aa
* add check_cycle.py * Update graphs/check_cycle.py Co-authored-by: John Law <johnlaw.po@gmail.com> * Update check_cycle.py * Apply suggestions from code review Co-authored-by: John Law <johnlaw.po@gmail.com> Co-authored-by: Christian Clauss <cclauss@me.com>
56 lines
1.5 KiB
Python
56 lines
1.5 KiB
Python
"""
|
|
Program to check if a cycle is present in a given graph
|
|
"""
|
|
|
|
|
|
def check_cycle(graph: dict) -> bool:
|
|
"""
|
|
Returns True if graph is cyclic else False
|
|
|
|
>>> check_cycle(graph={0:[], 1:[0, 3], 2:[0, 4], 3:[5], 4:[5], 5:[]})
|
|
False
|
|
>>> check_cycle(graph={0:[1, 2], 1:[2], 2:[0, 3], 3:[3]})
|
|
True
|
|
"""
|
|
# Keep track of visited nodes
|
|
visited = set()
|
|
# To detect a back edge, keep track of vertices currently in the recursion stack
|
|
rec_stk = set()
|
|
for node in graph:
|
|
if node not in visited:
|
|
if depth_first_search(graph, node, visited, rec_stk):
|
|
return True
|
|
return False
|
|
|
|
|
|
def depth_first_search(graph: dict, vertex: int, visited: set, rec_stk: set) -> bool:
|
|
"""
|
|
Recur for all neighbours.
|
|
If any neighbour is visited and in rec_stk then graph is cyclic.
|
|
|
|
>>> graph = {0:[], 1:[0, 3], 2:[0, 4], 3:[5], 4:[5], 5:[]}
|
|
>>> vertex, visited, rec_stk = 0, set(), set()
|
|
>>> depth_first_search(graph, vertex, visited, rec_stk)
|
|
False
|
|
"""
|
|
# Mark current node as visited and add to recursion stack
|
|
visited.add(vertex)
|
|
rec_stk.add(vertex)
|
|
|
|
for node in graph[vertex]:
|
|
if node not in visited:
|
|
if depth_first_search(graph, node, visited, rec_stk):
|
|
return True
|
|
elif node in rec_stk:
|
|
return True
|
|
|
|
# The node needs to be removed from recursion stack before function ends
|
|
rec_stk.remove(vertex)
|
|
return False
|
|
|
|
|
|
if __name__ == "__main__":
|
|
from doctest import testmod
|
|
|
|
testmod()
|