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..846d133 100644
--- a/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py
+++ b/Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py
@@ -8,13 +8,14 @@ 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
+    sorted_strings = sorted(strings)
+    for string_index in range(len(sorted_strings) - 1):
+        common = find_common_prefix(sorted_strings[string_index], sorted_strings[string_index + 1])
+        if len(common) > len(longest):
+            longest = common
     return longest
-
+# before changes, it was N^2, bacause it got a for loop inside a for loop
+# after changes, it is NlogN + N. The NlogN is the worst case for sorted, and another N for the loop.
 
 def find_common_prefix(left: str, right: str) -> str:
     min_length = min(len(left), len(right))
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..45e69e2 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.
     """
+    all_chars_set = set(s)
+    upper_set = set(letter for letter in all_chars_set if letter.isupper())
+    lower_set = set(letter for letter in all_chars_set if letter.islower())
+
     only_upper = set()
-    for letter in s:
-        if is_upper_case(letter):
-            if letter.lower() not in s:
-                only_upper.add(letter)
+    for letter in upper_set:
+        if letter.lower() not in lower_set:
+            only_upper.add(letter)
     return len(only_upper)
-
+#before: it was N^2. It took a for loop and a not in s, which is a loop.
+#after: it becomes N + N. The first N is the set created, it can access data instantly. The second N is the for loop.
 
 def is_upper_case(letter: str) -> bool:
     return letter == letter.upper()