125. Valid Palindrome

A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.

Given a string s, return true if it is a palindrome, or false otherwise.

'''
Careful with innner loop terminating condition

'''

def isPalindrome(s):

       

    L = len(s)

    left, right = 0, L - 1

   

    while left < right:

               

        while not s[left].isalnum() and left < right:

            left += 1

        while not s[right].isalnum() and right > left:

            right -= 1

           

        if s[left].lower() != s[right].lower():

            return False

       

        left  += 1

        right -= 1

       

    return True