# https://www.codewars.com/kata/recover-a-secret-string-from-random-triplets/train/python
# There is a secret string which is unknown to you. 
# Given a collection of random triplets from the string, recover the original string.
# A triplet here is defined as a sequence of three letters such that each letter occurs somewhere before the next in the given string. 
# "whi" is a triplet for the string "whatisup".
# As a simplification, you may assume that no letter occurs more than once in the secret string.

