input: Write a python function to find the longest shared substring between two given strings.
  assert longestSharedSubstring('thequickbrownfox', 'jumpedoverthelazydog') == 'the'
  assert longestSharedSubstring('abcdef', 'zabcxy') == 'abc'
  assert longestSharedSubstring('12345', '54321') == '1'
output: def longestSharedSubstring(s1, s2):
      def get_substrings(s):
          substrings = set()
          for i in range(len(s)):
              for j in range(i + 1, len(s) + 1):
                  substrings.add(s[i:j])
          return substrings
      
      substrings1 = get_substrings(s1)
      substrings2 = get_substrings(s2)
      
      common_substrings = substrings1.intersection(substrings2)
      
      if not common_substrings:
          return ''
      
      longest_common_substring = max(common_substrings, key=len)
      
      return longest_common_substring