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Bellman-ford.py
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Bellman-ford.py
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# Class to represent a graph
class Graph:
def __init__(self, vertices):
self.V = vertices # Total number of vertices in the graph
self.graph = [] # Array of edges
# Add edges
def addEdge(self, s, d, w):
self.graph.append([s, d, w])
# Print the solution
def print_solution(self, dist):
print("Vertex Distance from Source")
for i in range(self.V):
print("{0}\t\t{1}".format(i, dist[i]))
def BellmanFord(self, src, end):
pred = [None] * self.V
# Step 1: fill the distance array and predecessor array
dist = [float("Inf")] * self.V
# Mark the source vertex
dist[src] = 0
# Step 2: relax edges |V| - 1 times
for _ in range(self.V - 1):
for s, d, w in self.graph:
if dist[s] != float("Inf") and dist[s] + w < dist[d]:
dist[d] = dist[s] + w
pred[d] = s
# Step 3: detect negative cycle
# if value changes then we have a negative cycle in the graph
# and we cannot find the shortest distances
for s, d, w in self.graph:
if dist[s] != float("Inf") and dist[s] + w < dist[d]:
print("Graph contains negative weight cycle")
return
# No negative weight cycle found!
# Print the distance and predecessor array
self.print_solution(dist)
print(pred)
temp = end
route = [temp] # Set route to end
while pred[temp] is not None: # Iterate predecessor list to find path
route.append(pred[temp]) # Append path to route list from predecessor array
temp = pred[temp] # update temp end
route.reverse() # Reverse list to get right order
print(route)
# Driver's code
if __name__ == '__main__':
# g = Graph(5)
# g.addEdge(0, 1, -1)
# g.addEdge(0, 2, 4)
# g.addEdge(1, 2, 3)
# g.addEdge(1, 3, 2)
# g.addEdge(1, 4, 2)
# g.addEdge(3, 2, 5)
# g.addEdge(3, 1, 1)
# g.addEdge(4, 3, -3)
# g.BellmanFord(0)
#
g = Graph(7)
downhillScores = [(0, 6, -500), (1, 4, 100), (1, 2, 300),
(6, 3, -100), (6, 1, 200), (3, 4, 400), (3, 1, 400),
(5, 6, 700), (5, 1, 1000), (4, 2, 100)]
downhillScores.sort()
for trail in downhillScores:
g.addEdge(trail[0], trail[1], -trail[2])
#
# # function call
g.BellmanFord(6, 2)