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..f5284f4 100644
--- a/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py
+++ b/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py
@@ -7,18 +7,29 @@ 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.
     """
+    if len(strings) < 2:
+        return ""
+
+    sorted_strings = 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(len(sorted_strings) - 1):
+        common = find_common_prefix(
+            sorted_strings[i],
+            sorted_strings[i + 1]
+        )
+
+        if len(common) > len(longest):
+            longest = common
+
     return longest
 
 
 def find_common_prefix(left: str, right: str) -> str:
     min_length = min(len(left), len(right))
+
     for i in range(min_length):
         if left[i] != right[i]:
             return left[:i]
-    return left[:min_length]
+
+    return left[:min_length]
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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..49e5e21 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 +2,17 @@ def count_letters(s: str) -> int:
     """
     count_letters returns the number of letters which only occur in upper case in the passed string.
     """
+    letters_in_string = set(s)
+
     only_upper = set()
+
     for letter in s:
         if is_upper_case(letter):
-            if letter.lower() not in s:
+            if letter.lower() not in letters_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()
\ No newline at end of file