``````# Python Program to detect cycle in an undirected graph

from collections import defaultdict

#This class represents a undirected graph using adjacency list representation
class Graph:

def __init__(self,vertices):
self.V= vertices #No. of vertices
self.graph = defaultdict(list) # default dictionary to store graph

# function to add an edge to graph
self.graph[v].append(w) #Add w to v_s list
self.graph[w].append(v) #Add v to w_s list

# A recursive function that uses visited[] and parent to detect
# cycle in subgraph reachable from vertex v.
def isCyclicUtil(self,v,visited,parent):

#Mark the current node as visited
visited[v]= True

#Recur for all the vertices adjacent to this vertex
for i in self.graph[v]:
# If the node is not visited then recurse on it
if  visited[i]==False :
if(self.isCyclicUtil(i,visited,v)):
return True
# If an adjacent vertex is visited and not parent of current vertex,
# then there is a cycle
elif  parent!=i:
return True

return False

#Returns true if the graph contains a cycle, else false.
def isCyclic(self):
# Mark all the vertices as not visited
visited =[False]*(self.V)
# Call the recursive helper function to detect cycle in different
#DFS trees
for i in range(self.V):
if visited[i] ==False: #Don't recur for u if it is already visited
if(self.isCyclicUtil(i,visited,-1))== True:
return True

return False

# Create a graph given in the above diagram
g = Graph(5)

if g.isCyclic():
print "Graph contains cycle"
else :
print "Graph does not contain cycle "
g1 = Graph(3)