Valid Palindrome

Question

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example, "A man, a plan, a canal: Panama" is a palindrome. "race a car" is not a palindrome.

Tags

• String
• Two Pointers

Thought

Use the two pointers to start from the head and the tail of the string respectively and compare the characters at the corresponding position one by one.

Code

class Solution(object):
    def isPalindrome(self, s):
        """
        :type s: str
        :rtype: bool
        """
        l, r = 0, len(s)-1
        while l < r:
            if not s[l].isalnum():
                l += 1
            elif not s[r].isalnum():
                r -= 1
            else:
                if s[l].lower() != s[r].lower():
                    return False
                else:
                    l +=1; r -= 1
        return True