London | 25-SDC-July | Mikiyas Gebremichael | Sprint 2 | Improve code with precomputing

Sprint-2/improve_with_precomputing/common_prefix/common_prefix.py

@@ -1,24 +1,16 @@
 from typing import List
-
-
-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.
-
-    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
-    return longest
-
-
-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]
+def find_longest_common_prefix(strings: List[str]) -> str:
+    if len(strings) < 2:
+        return ""
+    #Find shortest string limit for prefix
+    min_len = min(len(s) for s in strings)
+    if min_len == 0:
+        return ""
+    #Compare character by character across all strings
+    for i in range(min_len):
+        char_set = set(s[i] for s in strings)
+        if len(char_set) > 1:  #mismatch found
+            return strings[0][:i]
+    return strings[0][:min_len]
+#Originally Compares every pair O(n² * m)
+#now compare column wise across all strings O(n * m)

Sprint-2/improve_with_precomputing/count_letters/count_letters.py

@@ -1,14 +1,15 @@
+from collections import Counter
 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:
-        if is_upper_case(letter):
-            if letter.lower() not in s:
-                only_upper.add(letter)
-    return len(only_upper)
-
-
-def is_upper_case(letter: str) -> bool:
-    return letter == letter.upper()
+    #Returns the number of letters which only occur in uppercase in the passed string
+    letters = Counter(s)  #count occurrences of each character
+    only_upper = 0
+    for letter in letters:
+        if letter.isupper():
+            if letter.lower() not in letters:
+                only_upper += 1
+    return only_upper
+#The first implementation was O(n² * m) in the worst case comparing every pair of strings character by character.
+# Now,
+#Counter scans the string once O(n)
+#Checking letter.lower() in letters O(1) per letter
+#Total time O(n + u) where u is the number of unique letters