Optimise count_letters: use Set for instant lookups

Author: AFatmaa

Sprint-2/improve_with_precomputing/count_letters/count_letters.py

@@ -2,13 +2,14 @@ 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()
+    # Create a Set of all characters in the string.
+    present_chars = set(s)
+    
+    only_upper_count = 0
+    # We loop through the unique characters
+    for char in present_chars:
+        if char.isupper():
+            if char.lower() not in present_chars:
+                only_upper_count += 1
+                
+    return only_upper_count