Improve performance of count_letters by avoiding repeated membership checks.

Author: eyuell21

Sprint-2/improve_with_precomputing/count_letters/count_letters.py

@@ -6,7 +6,7 @@ def count_letters(s: str) -> int:
 
     chars = set(s)  #store all characters once
 
-    for letter in s:
+    for letter in chars:
         if is_upper_case(letter):
             if letter.lower() not in chars:
                 only_upper.add(letter)