refactor: precomputing unique letters for faster lookup

Author: AhmadHmedann

Sprint-2/improve_with_precomputing/count_letters/count_letters.py

@@ -1,11 +1,27 @@
+# def count_letters(s: str) -> int:
+#     """
+#     count_letters returns the number of letters which only occur in upper case in the passed string.
+#     """
+#     only_upper = set()
+#     for letter in s:
+#         if is_upper_case(letter):
+#             if letter.lower() not in s:
+#                 only_upper.add(letter)
+#     return len(only_upper)
+
+
+# def is_upper_case(letter: str) -> bool:
+#     return letter == letter.upper()
+
 def count_letters(s: str) -> int:
     """
     count_letters returns the number of letters which only occur in upper case in the passed string.
     """
     only_upper = set()
-    for letter in s:
+    unique_letters = set(s)
+    for letter in unique_letters:
         if is_upper_case(letter):
-            if letter.lower() not in s:
+            if letter.lower() not in unique_letters:
                 only_upper.add(letter)
     return len(only_upper)