diff --git a/leetcode/17_letter_combinations_of_a_phone_number.py b/leetcode/17_letter_combinations_of_a_phone_number.py
index de6b40c..7e4fea5 100644
--- a/leetcode/17_letter_combinations_of_a_phone_number.py
+++ b/leetcode/17_letter_combinations_of_a_phone_number.py
@@ -1,21 +1,24 @@
+from typing import List
+
+
 class Solution:
     def letterCombinations(self, digits: str) -> List[str]:
         if len(digits) == 0:
             return []
 
         options = {
-            '2': ('a', 'b', 'c'),
-            '3': ('d', 'e', 'f'),
-            '4': ('g', 'h', 'i'),
-            '5': ('j', 'k', 'l'),
-            '6': ('m', 'n', 'o'),
-            '7': ('p', 'q', 'r', 's'),
-            '8': ('t', 'u', 'v'),
-            '9': ('w', 'x', 'y', 'z')
+            "2": ("a", "b", "c"),
+            "3": ("d", "e", "f"),
+            "4": ("g", "h", "i"),
+            "5": ("j", "k", "l"),
+            "6": ("m", "n", "o"),
+            "7": ("p", "q", "r", "s"),
+            "8": ("t", "u", "v"),
+            "9": ("w", "x", "y", "z"),
         }
 
         pools = (options[digit] for digit in digits)
         result = [""]
         for pool in pools:
-            result = [x+y for x in result for y in pool]
+            result = [x + y for x in result for y in pool]
         return result