diff --git a/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py b/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py
index f4839e7..5eb82f5 100644
--- a/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py
+++ b/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py
@@ -6,13 +6,19 @@ def find_longest_common_prefix(strings: List[str]):
     find_longest_common_prefix returns the longest string common at the start of any two strings in the passed list.
 
     In the event that an empty list, a list containing one string, or a list of strings with no common prefixes is passed, the empty string will be returned.
+    
+    Before changes: O(N^2 * avg(string_length)) - each string compared with each other string in double loop.
+    After changes: O(N * logN * avg(string_length))
+    
     """
+    if len(strings) <= 1:
+        return ""
+    strings.sort(reverse = True)
     longest = ""
-    for string_index, string in enumerate(strings):
-        for other_string in strings[string_index+1:]:
-            common = find_common_prefix(string, other_string)
-            if len(common) > len(longest):
-                longest = common
+    for index in range(len(strings) -1):
+        common = find_common_prefix(strings[index], strings[index+1])
+        if len(common) > len(longest):
+            longest = common
     return longest
 
 
diff --git a/Sprint-2/improve_with_precomputing/count_letters/count_letters.py b/Sprint-2/improve_with_precomputing/count_letters/count_letters.py
index 62c3ec0..4e114ee 100644
--- a/Sprint-2/improve_with_precomputing/count_letters/count_letters.py
+++ b/Sprint-2/improve_with_precomputing/count_letters/count_letters.py
@@ -1,13 +1,22 @@
 def count_letters(s: str) -> int:
     """
     count_letters returns the number of letters which only occur in upper case in the passed string.
+    
+    Before changes: O(N^2) - each letter compared with each other in double loop.
+    After changes: O(N + min(len(set_upper), min(len(set_lower)))
+    
     """
-    only_upper = set()
+    appeared_upper = set()
+    appeared_lower = set()
     for letter in s:
-        if is_upper_case(letter):
-            if letter.lower() not in s:
-                only_upper.add(letter)
-    return len(only_upper)
+        if not letter.isalpha():
+            continue
+        if letter.islower():
+            appeared_lower.add(letter.lower())
+        else:
+            appeared_upper.add(letter.lower())
+            
+    return len(appeared_upper.difference(appeared_lower))
 
 
 def is_upper_case(letter: str) -> bool:
diff --git a/Sprint-2/improve_with_precomputing/count_letters/count_letters_test.py b/Sprint-2/improve_with_precomputing/count_letters/count_letters_test.py
index fe88477..756d455 100644
--- a/Sprint-2/improve_with_precomputing/count_letters/count_letters_test.py
+++ b/Sprint-2/improve_with_precomputing/count_letters/count_letters_test.py
@@ -16,6 +16,9 @@ def test_both(self):
 
     def test_mixed(self):
         self.assertEqual(count_letters("aABCbcDEeFGhI"), 4)
+    
+    def test_non_alphabet(self):
+        self.assertEqual(count_letters("aABCbcDEeFGhI!!!"), 4)
 
     def test_long_string(self):
         s = ""