Optimized string is_palindrome main function (#688)

Co-authored-by: loan <[email protected]>

Author: chiralevy

algorithms/strings/is_palindrome.py

@@ -21,9 +21,9 @@ def is_palindrome(s):
     i = 0
     j = len(s)-1
     while i < j:
-        while i < j and not s[i].isalnum():
+        while not s[i].isalnum():
             i += 1
-        while i < j and not s[j].isalnum():
+        while not s[j].isalnum():
             j -= 1
         if s[i].lower() != s[j].lower():
             return False