# =-=-=-= DAY 55 [Easy] =-=-=-=
#
# Implement a URL shortener with the following methods:
# 
#  * shorten(url), which shortens the url into a six-character alphanumeric
#    string, such as zLg6wl.
#  * restore(short), which expands the shortened string into the original url. If
#    no such shortened string exists, return null.
# 
# Hint: What if we enter the same URL twice?
