Given an array A of strings made only from lowercase letters, return a list
of all characters that show up in all strings within the list (including
duplicates). For example, if a character occurs 3 times in all
strings but not 4 times, you need to include that character three times in the final
answer.
You may return the answer in any order.
Example 1:
Input: ["bella","label","roller"] Output: ["e","l","l"]
Example 2:
Input: ["cool","lock","cook"] Output: ["c","o"]
Note:
1 <= A.length <= 1001 <= A[i].length <= 100A[i][j] is a lowercase letter