Bellman ford python

Graph Algorithms/Bellman Ford/Python/BellmanFord.py

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+"""
+ author : Poonam-Mishra
+ Problem statement : Compute the shortest paths from a single source vertex to all other vertices in a weighted directed Graph
+ Caution : Bellman ford algorithm does not works for graph with negative weight cycles
+"""
+import sys
+infinity = sys.maxsize
+class Graph:
+    def __init__(self,numVertices):
+        self.numVert = numVertices
+        self.graph = []
+    def setEdge(self,start,end,weight):
+        self.graph.append((start,end,weight))
+    def BellmanFord(self,start):
+        distance = [infinity]*self.numVert
+        distance[start]=0
+
+        # Relax edges
+        for i in range(self.numVert-1):
+            for s,e,w in self.graph:
+                if distance[s] != infinity and distance[s] + w < distance[e]:
+                    distance[e]=distance[s]+w
+
+        # Check for negative weight cycle - Drawback of Bellman ford algorithm
+        for s,e,w in self.graph:
+            if distance[s] != infinity and distance[s] + w < distance[e]:
+                print ("The directed weighted graph contains negative weight cycle")
+                return
+
+        # Print distance of each vertex from source vertex
+        for i in range(self.numVert):
+            print("%d - %d" % (i, distance[i]))
+
+#Sample Input
+g = Graph(7)
+g.setEdge(0,1,6)
+g.setEdge(0,2,5)
+g.setEdge(0,3,5)
+g.setEdge(3,5,-1)
+g.setEdge(5,6,3)
+g.setEdge(4,6,3)
+g.setEdge(3,2,-2)
+g.setEdge(2,4,1)
+g.setEdge(2,1,-2)
+g.setEdge(1,4,-1)
+g.BellmanFord(0)
+
+#Sample Output
+"""
+0 - 0
+1 - 1
+2 - 3
+3 - 5
+4 - 0
+5 - 4
+6 - 3
+"""
+
+#Time Complexity
+"""
+|E| = no. of edges
+|V| = no. of vertices
+
+Best case TC :
+    TC = O(|E|*|V|-1) = O(n^2) [where n = number of vertices = number of edges]
+Worst case TC when the Graph is a complete graph
+    then |E| = n(n-1)/2
+    TC =  O((n(n-1)/2)*(n-1)) = O(n^3)
+"""
+
+
+#Space Complexity
+"""
+The Space complexity for the above algorithm is O(n) , where n is the number of vertices
+"""