perf: optimize count_letters via pre-computed character set

- Replace O(N) string scanning with O(1) set lookup
- Transition overall algorithm complexity from O(N^2) to O(N)
- Maintain legacy loop structure and helper functions for minimal code changes
- Add documentation explaining the pre-computing strategy

Author: Iswanna

Sprint-2/improve_with_precomputing/count_letters/count_letters.py

@@ -1,14 +1,23 @@
 def count_letters(s: str) -> int:
     """
     count_letters returns the number of letters which only occur in upper case in the passed string.
+    
+    Optimized via Pre-computing: We convert the string to a set upfront to make 
+    the 'not in' check a constant-time operation.
     """
+    # PRE-COMPUTING: Create a set of all unique characters in the string.
+    # Searching a set takes O(1) time, while searching a string takes O(N) time.
+    chars_in_string = set(s)
+
     only_upper = set()
+    # LEGACY LOOP: Iterate through the string as originally designed.
     for letter in s:
         if is_upper_case(letter):
-            if letter.lower() not in s:
+            # OPTIMIZATION: Check against the pre-computed set instead of the full string 's'.
+            if letter.lower() not in chars_in_string:
                 only_upper.add(letter)
     return len(only_upper)
 
 
 def is_upper_case(letter: str) -> bool:
-    return letter == letter.upper()
+    return letter == letter.upper()