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..14d9069 100644
--- a/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py
+++ b/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py
@@ -7,12 +7,14 @@ def find_longest_common_prefix(strings: List[str]):
 
     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.
     """
+    strings_sorted = sorted(strings)
     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 i in range(1, len(strings_sorted)):
+        string = strings_sorted[i - 1]
+        other_string = strings_sorted[i]
+        common = find_common_prefix(string, other_string)
+        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..7ad1591 100644
--- a/Sprint-2/improve_with_precomputing/count_letters/count_letters.py
+++ b/Sprint-2/improve_with_precomputing/count_letters/count_letters.py
@@ -2,13 +2,12 @@ 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()
+    only_upper = 0
+    only_uniq = set(s)
+    only_uniq_sorted = sorted(only_uniq)
+    uppercase = [ch for ch in only_uniq_sorted if ch.isupper()]
+    lowercase = [ch for ch in only_uniq_sorted if ch.islower()]
+    for i in uppercase:
+        if i.lower() not in lowercase:
+            only_upper += 1
+    return only_upper