WestMidlands| SDC NOV 2025 | Sara Tahir | Sprint 2 |Improve with precomputing

Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py

@@ -3,22 +3,36 @@
 
 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.
+    Return the longest common prefix shared by any two strings in the 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 the list is empty, contains only one string, or no common prefixes
+    exist, an empty string is returned.
     """
+    if len(strings) < 2:
+        return ""
+
+    # Sorting places strings with similar prefixes next to each other.
+    # This reduces comparisons from O(n^2) to O(n log n).
+    strings.sort()
+
     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(strings) - 1):
+        # Compare only adjacent strings after sorting.
+        common = find_common_prefix(strings[i], strings[i + 1])
+        if len(common) > len(longest):
+            longest = common
+
     return longest
 
 
 def find_common_prefix(left: str, right: str) -> str:
+    """
+    Return the common prefix between two strings.
+    """
     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]

Sprint-2/improve_with_precomputing/count_letters/count_letters.py

@@ -2,13 +2,28 @@ 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()
+    lower_case = set()
+    upper_case= set()
+
+
+## Precompute lowercase and uppercase letters
     for letter in s:
-        if is_upper_case(letter):
-            if letter.lower() not in s:
-                only_upper.add(letter)
-    return len(only_upper)
+        if letter.islower():
+            lower_case.add(letter)
+        elif letter.isupper():
+            upper_case.add(letter)
+
+
+# Count uppercase letters whose lowercase version never appears
+    count = 0
+    for letter in upper_case:
+        if letter.lower() not in lower_case:
+            count += 1
+    return count
 
 
 def is_upper_case(letter: str) -> bool:
     return letter == letter.upper()
+
+#The original solution was O(n²) because it repeatedly checked membership in the full string.
+#By precomputing lowercase and uppercase sets, membership checks become O(1), so the whole function becomes O(n).