每日算法题 - Find Cycles in a Graph

Hi, here's your problem today. This problem was recently asked by Facebook:

Given an undirected graph, determine if a cycle exists in the graph.

Here is a function signature:

def find_cycle(graph):

  # Fill this in.



graph = {

  'a': {'a2':{}, 'a3':{} },

  'b': {'b2':{}},

  'c': {}

}

print find_cycle(graph)

# False

graph['c'] = graph

print find_cycle(graph)

# True

Can you solve this in linear time, linear space?