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..3af9ceb 100644
--- a/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py
+++ b/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py
@@ -1,24 +1,32 @@
 from typing import List
+from functools import lru_cache
 
 
-def find_longest_common_prefix(strings: List[str]):
-    """
-    find_longest_common_prefix returns the longest string common at the start of any two strings in the passed list.
+def find_longest_common_prefix(strings: List[str]) -> str:
+    if len(strings) < 2:
+        return ""
+
+    # KEY INSIGHT: sort the strings. The longest common prefix between
+    # any two strings must appear between two adjacent strings when sorted.
+    # So we only need to check n-1 pairs instead of n² pairs.
+    sorted_strings = sorted(strings)
 
-    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
+    for i in range(len(sorted_strings) - 1):
+        common = cached_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:
+def cached_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]
+
+
+def find_common_prefix(left: str, right: str) -> str:
+    return cached_find_common_prefix(left, right)
\ No newline at end of file
diff --git a/Sprint-2/improve_with_precomputing/common_prefix/common_prefix_test.py b/Sprint-2/improve_with_precomputing/common_prefix/common_prefix_test.py
index a22ef79..90f031d 100644
--- a/Sprint-2/improve_with_precomputing/common_prefix/common_prefix_test.py
+++ b/Sprint-2/improve_with_precomputing/common_prefix/common_prefix_test.py
@@ -1,7 +1,6 @@
 import random
 import string
 import unittest
-
 from common_prefix import find_longest_common_prefix
 
 class CommonPrefixTest(unittest.TestCase):
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..d5f3434 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.
     """
+    # Precompute: build a set of all characters ONCE — lookups are now O(1)
+    char_set = set(s)
+
     only_upper = set()
-    for letter in s:
+    for letter in char_set:            # iterate unique chars only, not all 10M
         if is_upper_case(letter):
-            if letter.lower() not in s:
+            if letter.lower() not in char_set:  # O(1) lookup instead of O(n)
                 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
diff --git a/Sprint-2/improve_with_precomputing/count_letters/count_letters_test.py b/Sprint-2/improve_with_precomputing/count_letters/count_letters_test.py
index fe88477..9024cf7 100644
--- a/Sprint-2/improve_with_precomputing/count_letters/count_letters_test.py
+++ b/Sprint-2/improve_with_precomputing/count_letters/count_letters_test.py
@@ -1,7 +1,6 @@
 import random
 import string
 import unittest
-
 from count_letters import count_letters
 
 class CommonPrefixTest(unittest.TestCase):