Optimize count_letters using precomputed character set for O(n) performance

Author: PratiAmalden

Sprint-2/improve_with_precomputing/count_letters/count_letters.py

@@ -2,13 +2,11 @@ def count_letters(s: str) -> int:
     """
     count_letters returns the number of letters which only occur in upper case in the passed string.
     """
-    only_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)
+    chars = set(s)  # precompute all characters once
 
+    only_upper = set()
+    for letter in chars:  # iterate unique characters only
+        if letter.isupper() and letter.lower() not in chars:
+            only_upper.add(letter)
 
-def is_upper_case(letter: str) -> bool:
-    return letter == letter.upper()
+    return len(only_upper)