fix solution: update comment

Author: dusunax

valid-palindrome/dusunax.py

@@ -1,7 +1,7 @@
 '''
 # Leetcode 125. Valid Palindrome
 
-use regex to filter out non-alphanumeric characters 🔍
+use `isalnum()` to filter out non-alphanumeric characters 🔍
 
 ## Time and Space Complexity
 
@@ -11,10 +11,12 @@
 ```
 
 ### TC is O(n):
-- iterating through the string just once to filter out non-alphanumeric characters.
+- iterating through the string just once to filter out non-alphanumeric characters. O(n)
 
 ### SC is O(n):
-- creating a new string to store the filtered characters.
+- `s.lower()` creates a new string. O(n)
+- creating a new string `converted_s` to store the filtered characters. O(n)
+- `converted_s[::-1]` creates a new reversed string. O(n)
 '''
 
 class Solution: