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105 lines
2.8 KiB
Python
105 lines
2.8 KiB
Python
# Algorithm from GeeksforGeeks (wrong)
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# https://www.geeksforgeeks.org/tree-back-edge-and-cross-edges-in-dfs-of-graph/
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import random
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class Graph:
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# instance variables
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def __init__(self, v):
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# v is the number of nodes/vertices
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self.time = 0
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self.traversal_array = []
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self.v = v
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# e is the number of edge (randomly chosen between 9 to 45)
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self.e = random.randint(9, 45)
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# adj. list for graph
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self.graph_list = [[] for _ in range(v)]
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# adj. matrix for graph
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self.graph_matrix = [[0 for _ in range(v)] for _ in range(v)]
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# function to create random graph
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def create_random_graph(self):
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# add edges upto e
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for i in range(self.e):
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# choose src and dest of each edge randomly
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src = random.randrange(0, self.v)
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dest = random.randrange(0, self.v)
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# re-choose if src and dest are same or src and dest already has an edge
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while src == dest and self.graph_matrix[src][dest] == 1:
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src = random.randrange(0, self.v)
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dest = random.randrange(0, self.v)
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# add the edge to graph
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self.add_edge(src, dest)
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def add_edge(self, src, dest):
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self.graph_list[src].append(dest)
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self.graph_matrix[src][dest] = 1
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# function to print adj list
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def print_graph_list(self):
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print("Adjacency List Representation:")
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for i in range(self.v):
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print(i, "-->", *self.graph_list[i])
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print()
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# function to print adj matrix
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def print_graph_matrix(self):
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print("Adjacency Matrix Representation:")
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for i in self.graph_matrix:
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print(i)
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print()
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# function the get number of edges
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def number_of_edges(self):
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return self.e
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# function for dfs
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def dfs(self):
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self.visited = [False]*self.v
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self.start_time = [0]*self.v
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self.end_time = [0]*self.v
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for node in range(self.v):
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if not self.visited[node]:
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self.traverse_dfs(node)
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print()
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print("DFS Traversal: ", self.traversal_array)
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print()
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def traverse_dfs(self, node):
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self.visited[node] = True
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self.traversal_array.append(node)
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self.start_time[node] = self.time
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self.time += 1
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for neighbour in self.graph_list[node]:
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print('Edge:', str(node)+'-->'+str(neighbour))
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if not self.visited[neighbour]:
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print(' => Tree Edge')
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self.traverse_dfs(neighbour)
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else:
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print(f"Times: ({self.start_time[node]}, {self.end_time[node]}) ({self.start_time[neighbour]}, {self.end_time[neighbour]})")
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if self.start_time[node] > self.start_time[neighbour] and self.end_time[node] < self.end_time[neighbour]:
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print(' => Back Edge')
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elif self.start_time[node] < self.start_time[neighbour] and self.end_time[node] > self.end_time[neighbour]:
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print(' => Forward Edge')
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else:
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print(' => Cross Edge')
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self.end_time[node] = self.time
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self.time += 1
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if __name__ == "__main__":
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g = Graph(4)
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# g.create_random_graph()
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g.add_edge(0, 1)
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g.add_edge(1, 2)
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g.add_edge(2, 3)
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g.add_edge(3, 1)
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g.print_graph_list()
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g.print_graph_matrix()
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g.dfs()
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