collect all lowercase letters present in s string into a set beforehand

Author: HassanOHOsman

Sprint-2/improve_with_precomputing/count_letters/count_letters.py

@@ -1,7 +1,11 @@
 def count_letters(s: str) -> int:
-    """
-    count_letters returns the number of letters which only occur in upper case in the passed string.
-    """
+    only_lower = set()
+    for letter in s:
+        if letter.islower():
+            only_lower.add(letter)
+
+
+
     only_upper = set()
     for letter in s:
         if is_upper_case(letter):