As we saw with the merge point problem, more than one node can reference another node. These references can create a cycle in the linked list where the traversal will loop back on itself.

#    -> b -> c
#  /      \   \
# a         d <-

# 'd' node's next points to 'b' node

Write a function that detects whether a cycle exists in a linked list. A cycle exists if traversing the linked list visits the same node more than once.

A cycle does not mean repeated values. Avoid this pitfall in your implementation by comparing the Node instances themselves, not their values!

a = Node('a')
other_a = Node('a')

a.val == other_a.val 
# True
a == other_a
# False

To recap:

• write a function: has_cycle().
• has_cycle() takes an instance of LinkedList as the argument.
• return a Boolean which indicates whether a cycle exists.