Valid Palindrome

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

Note: For the purpose of this problem, we define empty string as valid palindrome.

Example 1:

Input:
 "A man, a plan, a canal: Panama"

Output:
 true

Example 2:

Input:
 "race a car"

Output:
 false

分析

考虑只有字母 isalnum 和大小写

别忘了lower case!!!!!

class Solution:
    def isPalindrome(self, s):
        """
        :type s: str
        :rtype: bool
        """
        s = [c.lower() for c in s if c.isalnum()]
        start, end = 0, len(s) - 1
        while start < end:
            if s[start] != s[end]:
                return False
            start += 1
            end -= 1
        return True

class Solution:
    def isPalindrome(self, s):
        """
        :type s: str
        :rtype: bool
        """
        if not s:
            return True
        start, end = 0, len(s) - 1
        while start < end:
            while start < end and not s[start].isalnum():
                start +=1
            while start < end and not s[end].isalnum():
                end-=1

            if s[start].lower() != s[end].lower():
                return False
            start += 1
            end -= 1
        return True