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:

<pre lang=py>
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
</pre>

Can you solve this in linear time, linear space?