599 Minimum Index Sum of Two Lists

Suppose Andy and Doris want to choose a restaurant for dinner, and they both have a list of favorite restaurants represented by strings.
You need to help them find out their common interest with the least list index sum. If there is a choice tie between answers, output all of them with no order requirement. You could assume there always exists an answer.
Example 1:
["Shogun", "Tapioca Express", "Burger King", "KFC"]
["Piatti", "The Grill at Torrey Pines", "Hungry Hunter Steakhouse", "Shogun"]
The only restaurant they both like is "Shogun".
Example 2:
["Shogun", "Tapioca Express", "Burger King", "KFC"]
["KFC", "Shogun", "Burger King"]
The restaurant they both like and have the least index sum is "Shogun" with index sum 1 (0+1).
  1. 1.
    The length of both lists will be in the range of [1, 1000].
  2. 2.
    The length of strings in both lists will be in the range of [1, 30].
  3. 3.
    The index is starting from 0 to the list length minus 1.
  4. 4.
    No duplicates in both lists.
The Idea: Aggregate the the sum of indices that share common interests into a list. Then all the remains will be to return the appropriate list that is mapped from the lowest sum of indices.
Complexity: O(n) time and space
import collections
class Solution:
def findRestaurant(self, list1, list2):
:type list1: List[str]
:type list2: List[str]
:rtype: List[str]
fav2 = {word:i for i, word in enumerate(list2)}
ties_dictionary = collections.defaultdict(list)
for i, restaurant in enumerate(list1):
if restaurant in fav2:
return min(ties_dictionary.items(), key=lambda x: x[0])[1]