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..0d769d7 100644
--- a/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py
+++ b/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py
@@ -7,13 +7,17 @@ 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.
     """
-    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
-    return longest
+        if len(strings) < 2:
+            return ""
+
+        sorted_strings = sorted(strings)
+        first = sorted_strings[0]
+        last = sorted_strings[-1]
+        min_length = min(len(first), len(last))
+        i = 0
+        while i < min_length and first[i] == last[i]:
+            i += 1
+        return first[:i]
 
 
 def find_common_prefix(left: str, right: str) -> str:
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..a413edc 100644
--- a/Sprint-2/improve_with_precomputing/count_letters/count_letters.py
+++ b/Sprint-2/improve_with_precomputing/count_letters/count_letters.py
@@ -1,14 +1,9 @@
 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()
+    lowers = set(ch for ch in s if ch.islower())
+    uppers = set(ch for ch in s if ch.isupper())
+    count = 0
+    for ch in uppers:
+        if ch.lower() not in lowers:
+            count += 1
+    return count