Replace Counter with set in count_letters for efficiency and update comment

Author: Mikiyas-STP

Sprint-2/improve_with_precomputing/count_letters/count_letters.py

@@ -1,15 +1,13 @@
 from collections import Counter
 def count_letters(s: str) -> int:
     #Returns the number of letters which only occur in uppercase in the passed string
-    letters = Counter(s)  #count occurrences of each character
+    letters = set(s)
     only_upper = 0
     for letter in letters:
-        if letter.isupper():
-            if letter.lower() not in letters:
+        if letter.isupper() and letter.lower() not in letters:
                 only_upper += 1
     return only_upper
-#The first implementation was O(n² * m) in the worst case comparing every pair of strings character by character.
-# Now,
-#Counter scans the string once O(n)
-#Checking letter.lower() in letters O(1) per letter
-#Total time O(n + u) where u is the number of unique letters
+# Convert the string to a set to remove duplicate characters in O(n)
+# Iterate over unique letters and check lowercase membership in O(1)
+# Total time complexity: O(n)
+# Space complexity: O(u), where u is the number of unique characters