In “Find the Longest Repeat in a String”1, we encountered the Longest Repeat Problem, which could be solved using a suffix-tree.

. A third such problem is shown below.

Assignment

Implement the shortest_unique_substring method which takes 2 strings.

The function should return a shortest substring of the given strings that occurs in only one of them.

Example

>>> shortest_unique_substring('GGGTACCTCGCG', 'CCCCGGTTGCCA')
'CA'