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* Type annotations for `breadth_first_search.py` * Type annotations for `breadth_first_search_2.py` * Remove from excluded in mypy.ini * Add doctest.testmod() * Type annotations for `graphs/check_cycle.py` * Type annotations for `graphs/greedy_min_vertex_cover.py` * Remove from excluded in mypy.ini
54 lines
1.5 KiB
Python
54 lines
1.5 KiB
Python
"""
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Program to check if a cycle is present in a given graph
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"""
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def check_cycle(graph: dict) -> bool:
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"""
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Returns True if graph is cyclic else False
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>>> check_cycle(graph={0:[], 1:[0, 3], 2:[0, 4], 3:[5], 4:[5], 5:[]})
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False
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>>> check_cycle(graph={0:[1, 2], 1:[2], 2:[0, 3], 3:[3]})
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True
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"""
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# Keep track of visited nodes
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visited: set[int] = set()
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# To detect a back edge, keep track of vertices currently in the recursion stack
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rec_stk: set[int] = set()
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for node in graph:
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if node not in visited:
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if depth_first_search(graph, node, visited, rec_stk):
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return True
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return False
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def depth_first_search(graph: dict, vertex: int, visited: set, rec_stk: set) -> bool:
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"""
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Recur for all neighbours.
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If any neighbour is visited and in rec_stk then graph is cyclic.
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>>> graph = {0:[], 1:[0, 3], 2:[0, 4], 3:[5], 4:[5], 5:[]}
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>>> vertex, visited, rec_stk = 0, set(), set()
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>>> depth_first_search(graph, vertex, visited, rec_stk)
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False
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"""
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# Mark current node as visited and add to recursion stack
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visited.add(vertex)
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rec_stk.add(vertex)
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for node in graph[vertex]:
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if node not in visited:
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if depth_first_search(graph, node, visited, rec_stk):
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return True
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elif node in rec_stk:
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return True
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# The node needs to be removed from recursion stack before function ends
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rec_stk.remove(vertex)
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return False
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if __name__ == "__main__":
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from doctest import testmod
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testmod()
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