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..b9865a0 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 +7,13 @@ 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.
     """
+    seen = set()
     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
-
+    for string in strings:
+        for i in range(1, len(string) + 1):
+            prefix = string[:i]
+            if prefix in seen and len(prefix) > len(longest):
+                longest = prefix
 
-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]
+            seen.add(prefix)
+    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..d33b474 100644
--- a/Sprint-2/improve_with_precomputing/count_letters/count_letters.py
+++ b/Sprint-2/improve_with_precomputing/count_letters/count_letters.py
@@ -3,9 +3,10 @@ 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:
+    unique_letters = set(s)
+    for letter in unique_letters:
         if is_upper_case(letter):
-            if letter.lower() not in s:
+            if letter.lower() not in unique_letters:
                 only_upper.add(letter)
     return len(only_upper)