Optimize letter counting by using pre-computed character set

Author: eyuell21

Sprint-2/improve_with_precomputing/count_letters/count_letters.py

@@ -3,12 +3,16 @@ 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()
+    
+    chars = set(s)  #store all characters once
+
     for letter in s:
         if is_upper_case(letter):
-            if letter.lower() not in s:
+            if letter.lower() not in chars:
                 only_upper.add(letter)
+
     return len(only_upper)
 
 
 def is_upper_case(letter: str) -> bool:
-    return letter == letter.upper()
+    return letter == letter.upper()