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..96b633e 100644
--- a/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py
+++ b/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py
@@ -1,18 +1,26 @@
 from typing import List
 
 
-def find_longest_common_prefix(strings: List[str]):
+def find_longest_common_prefix(strings: List[str]) -> 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.
     """
+    if len(strings) < 2:
+        return ""
+
+    # PRE-COMPUTE by sorting strings once
+    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
+
+    #compare only strings that are next to eacotherr in sorted order
+    for i in range(len(strings) - 1):
+        common = find_common_prefix(strings[i], strings[i + 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..e52245a 100644
--- a/Sprint-2/improve_with_precomputing/count_letters/count_letters.py
+++ b/Sprint-2/improve_with_precomputing/count_letters/count_letters.py
@@ -3,12 +3,16 @@ 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:
+    
+    chars = set(s)  #store all characters once
+
+    for letter in chars:
         if is_upper_case(letter):
-            if letter.lower() not in s:
+            if letter.lower() not in chars:
                 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