Performance: Remove unnecessary sorting from is_isogram()

Author: Milad Khoshdel

strings/is_isogram.py

@@ -19,7 +19,7 @@ def is_isogram(string: str) -> bool:
     if not all(x.isalpha() for x in string):
         raise ValueError("String must only contain alphabetic characters.")
 
-    letters = sorted(string.lower())
+    letters = string.lower()
     return len(letters) == len(set(letters))