Nodes within a linked list can be referenced multiple times. We’ll explore this idea with a partially merged linked list.

# a -> b
#        \
#         -> c -> e
#        /
# d -> f

In this example, two different heads (nodes holding 'a' and 'd') merge into a single linked list with the node holding 'c'. This “merge point” results from nodes holding 'b' and 'f' both referencing the node holding 'c' as the .next property.

Write a function that returns the merge point node of two linked lists if it exists.

# x -> a -> b
#            \
#             -> q -> e
#            /
#      d -> f

# node holding 'q'

# r
#  \
#   -> x
#  /
# f

# node holding 'x'

# j -> k
# l -> q

# None

To recap:

• write a function: merge_point().
• merge_point() takes two arguments, both instances of a linked list.
• return the first node referenced by both linked lists, or None if such a node does not exist.