Optimise count_letters by precomputing lowercase/uppercase sets

Author: SaraTahir28

Sprint-2/improve_with_precomputing/count_letters/count_letters.py

@@ -2,13 +2,28 @@ 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()
+    lower_case = set()
+    upper_case= set()
+
+
+## Precompute lowercase and uppercase letters
     for letter in s:
-        if is_upper_case(letter):
-            if letter.lower() not in s:
-                only_upper.add(letter)
-    return len(only_upper)
+        if letter.islower():
+            lower_case.add(letter)
+        elif letter.isupper():
+            upper_case.add(letter)
+
+
+# Count uppercase letters whose lowercase version never appears
+    count = 0
+    for letter in upper_case:
+        if letter.lower() not in lower_case:
+            count += 1
+    return count
 
 
 def is_upper_case(letter: str) -> bool:
     return letter == letter.upper()
+
+#The original solution was O(n²) because it repeatedly checked membership in the full string.
+#By precomputing lowercase and uppercase sets, membership checks become O(1), so the whole function becomes O(n).